Command & Conquer: Red Alert 3/Scouting

Scouting is a tactic that is used to gain intelligence on an enemy, including troop movements, potential targets, finding weak points in an enemy base, or just knowing what your enemy is up to.

Scouting is important at all times, not just early-game. Some strategies are easy to counter, but require a very specific counter that might take too long to build if the attack isn't spotted early enough. It is also very important to know how many Refineries your opponent has built so you don't fall behind economically.

While some units are very good at scouting, any unit can do the job. In some situations all that's needed is visibility to know if an opponent has taken an oil or expanded to another location. Also, most maps have neutral structures in the middle, which, when garrisoned with any infantry, will provide a passive scout as tough as the structure. Finally, the neutral Observation Tower provides a large field of vision and has high health.

Allies
Attack Dogs are the Allies' most obvious scout. They have the largest vision range of all Allied units besides the MCV and are fast an inexpensive. Dogs are the standard option for scouting the map at the beginning of the game, but are less effective against another Allied player using Peacekeepers. Dogs retain their scouting utility through out most of the game because they are so inexpensive. Finding points where dogs can hide idle on the map is key for passive scouting.

Toward the middle of the game, Apollos become a great option for scouting. They are fast, have much more health and can traverse the whole map. Apollos are great for checking multiple expansion points in one run while also checking key details of the opponent's base. Because they're much more expensive, Apollos should only be used for quick checks and then moved to a safe spot on the map.

Late game, a Mirage Tank can provide a permanent view of an area until it's detected. Surveillance Sweep can also be used for scouting, but isn't available at all times and require that you spend a Player Point on the upgrade. Finally, infiltrating an enemy MCV with a Spy will reveal what that MCV can see for the rest of the game, which a powerful scouting tool.

Soviets
War Bears are the Soviet counterpart to the Allied Attack Dog. Because they have much more health, Bears have more utility in combat, meaning that they're more likely to be built at any given time and ready to use as scouts than the dogs. Forgoing the Barracks, Terror Drones perform much the same function while providing a threat for enemy vehicles.

Sickles can be used for scouting in some situations when attacking with them isn't the best option. With their Flea Jump, they maneuver onto cliffs where lower ground forces can't see them. For just the scouting utility though, it may be better to use a Bullfrog's Mancannon to send a bear to the same position for less money and wider vision. In this manner, bears may be used similar to a flare.

When the Soviets finally obtain air units, the MiG can be used for scouting in much the same way as the Allied Apollo.

Finally, the Akula Submarine, set to hold fire, can be a good scout on areas in and near water because they won't show up on radar and the enemy won't be alerted to their presence until they're fired upon.

Empire of the Rising Sun
Burst Drones are the Empire dedicated scout. Unlike the Attack Dog and the War Bear, Burst Drones can fly, ignoring all of the map's obstacles. It can attach to harvesters to slow them down as long as they aren't noticed.

Mecha Tengu squadrons are Empire's best scouting option because they can strike vulnerable areas as soon as they're discovered.

Yari Mini-Subs can be used in a similar way to the Soviet Akula Submarine.

Like Allies, Empire can infiltrate MCVs using their Shinobi unit. Also, Sudden Transports can hide as a disguised unit in an opponent's base.

Lastly, an advanced Empire scouting technique to just track a land-based refinery expansion is to send a Dojo Core to the expansion early on. Walls can be built where the refinery would be put, preventing ideal refinery placement until the walls are dealt with, which will be noticed.