Overview
Scouting is an inherent part of the game, and essential for any sort of progress in skill. Knowledge of your enemy's actions is vital to making good and effective decisions throughout the game. Often, when asking a more experienced player how they should improve, the answer is 'scout more' or 'scout better'. However, what does this mean exactly?
Just blindly sending your units out to random positions on the map serves little purpose. So here are a few pointers on when and how to scout, and most importantly, why it's important in the first place.
Purposes for Scouting:
1. Build Order
At the beginning of the game, you know nothing about your enemy. After your initial scout finds the enemy, try to stay in his base for as long as possible to try to find out what his build order is. Send scouts periodically during the beginning phase of the game so you know what you are up against.
A very simple example is Zerg versus Protoss. With your initial scout (either drone or overlord), if you see a double-gateway build, you know he is rushing or building up an early attack force. Whereas if you see single gateway to cybernetics core, you can deduce that he is teching. Without this information, you would be playing blind and trying to guess at his plans.
The good thing about build orders is that once you reach a certain (low-moderately skilled) level of play, it is not difficult to figure out what your enemy is doing, as faking a build order to provide you with false information is usually not worth the effort.
Later on, scouting an opponent's base for tech buildings allows you to prepare in advance for your opponent's units. For example, if you spot a Greater Hive, you can expect either Guardians or Devourers to make an appearance.
2. Army Position and Composition
The location of your opponent's army is very important. Armies are generally mobile, and in the later stages of the game it becomes increasingly difficult to defend all of your expansions well. Thus, knowing the location of your opponent's army will allow you to place your forces accordingly - either defensively, against his attack, or offensively, striking a hole in his defenses.
In addition to location, the composition of the army is also important. Once a player gains enough knowledge of the game mechanics, knowing the effective counters to various units becomes natural, and the player can prepare in advance for what he knows his enemy has.
3. Scouting Expansions
Possibly one of the most important functions of scouting is to discover your opponent's expansions (or desire to expand) early, and thus enable you to stop them. An expansion is most vulnerable when it is in the process of being built, or just being built. Once an expo is established, it is significantly easier for the defending player to defend it. In higher level games, this type of scouting occurs frequently, as an extra expansion is often enough to tip the balance for the player to win the game.
When to Scout:
This is a very difficult question to answer. The simple answer would be 'all the time', but of course its impractical and inefficient to follow this doctrine. A more accurate but equally useless answer is 'you will eventually feel when it is the right time to scout'. At a certain level of skill, you will gain a gut feeling that tells you when and where to scout. Things that your brain pieces together behind the scenes while you are busy making decisions, will be evaluated by your brain and then bubble up as a thought. "He has a small army, and he isn't teching hard. He must be expanding. Go scout it out", "His army at his main base is too small, he must be setting up an attack", are examples of how you can evaluate the present circumstances in the game to come to conclusions on your enemy's actions BEFORE sending out a scout. This is learned through experience and observation.
Before you gain this sense though, there are several key points in the game when you should scout.
1. At the beginning. Discover your enemy's location and his build order. 2. When you finish your first tech (Tanks, Lurkers, High templar/Reaver/Corsair). Your opponent's tech choice will be coming into play at around this time as well. 3. Whenever you are not in a major battle, after the first big battle of the game. Search for new expansions.
These are just guidelines that will help put some structure in the 'scout all the time' doctrine.
How to Scout:
This will be covered in more detail in the entries for specific races.
It is impossible to write down all the information which can be gathered by scouting, and the appropriate actions to take in response to your new knowledge. However, time and experience will help you develop the sense of when and where to scout. In the meantime, use this guide to help you focus your efforts in improving your game.
Terran
The Terrans are at a disadvantage in the early game for scouting, primarily because their Marines are needed at base to defend against the faster and more flexible Zerglings. Shortly afterward, however, Terran Firebats and Stim-Pack become slightly more effective. Scouting is also of less importance for Terrans early game, because the Marines can resist both ground and air attacks and ComSat Station, which provides detection coverage, also arrives early in the game.
After the construction of the Factory, players can choose to either Addon Machine Shop to quickly gain the important associated upgrades, or first produce a Vulture. Vulture speed-bikes are well, fast, and even faster with the upgrade, making them great for scouting. In fact, they may scout deep into an enemy base and back out without being destroyed once the player has the Ion Engines upgrade. Vultures serve a triple purpose: 1) their attacks do excellent damage against Zerglings and they are fast enough for successful hit-and-run tactics against even speed-upgraded Zerglings, who still use melee; 2) they are excellent scouts, especially with the speed upgrade; and 3) they each have three Spider Mines, which may be sowed at bottleneck locations to impede enemy attacks, at enemy base entrances to monitor their activity, or at optimal main-building construction sites to destroy the enemy attempt to expand.
After the construction of the Academy, players can choose to Addon ComSat Station, which grants a temporary view and detection of a target location. Judicious use of ComSat can provide uncontested view of an enemy base without placing any units at risk.
Late-game reconnaisance is mainly through the Wraith, which can become invisible for an energy cost.
Zerg
At the beginning of the game, the Zerg Overlord can immediately be sent on scouting missions, though this is better for discovering the location of the enemy than for learning the complexity of the enemy base because the Overlords are slow and form a critical asset early in the game.
By far the best Zerg scout is the Zergling. Like the Terran Vulture, the Zerglings are swift; however, they are also cheap, which makes using them for reconnaisance a cheap endeavor. Their ability to burrow allows them to spring surprise attacks on enemies as well as provide cheap sentry of an area. Players may burrow them at optimal base locations to 1) detect and counter enemy attempts at expansion; and 2) by existing at the location, preventing the enemy from building its main-base structure (Command Center, Hatchery, Nexus). The fact that Zerglings can be split into two for just 50 minerals makes them a particularly valuable asset; their ability to quickly deal 60 damage makes them effective against SCV's, Drones, and Probes. All other Zerg units are simply less effective at providing reconnaisance than the Zergling.
The Queen's Parasite ability allows the Zerg to create a scouting unit from an enemy unit, even if the enemy unit moves. As a result, the Zerg may have good advance warning of an enemy movement.
Protoss
After the Leg Enhancements upgrade from the Citadel of Adun, Zealots become modest scouts, able to chase after Zerglings effectively and able to penetrate deep into the enemy base due to their significant shield and life points.
With the advent of Hallucination and High Templars, units may be hallucinated and the dittoes may be sent on scouting missions. Hallucinations perform a double task: 1) they distract enemies; and 2) they are a resource-expenditure-free means of scouting the enemy. Hallucinations of the Shuttle or Corsair, the fastest units in the Protoss arsenal and both aerial, may be even more wieldy for scouting purposes.
Observers are by far the most useful and unexpected scouting unit for any race. Combining speed (through its upgrade), detection, and permanent invisibility for itself, the Observer can detect enemy movements and structure/technology patterns without exposing its presence, which can trick an enemy without detection into a false sense of secrecy and security.