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 can be used anytime, not just early-game.

Allies
Early-game, the Allies can send Attack Dogs to an enemy base for scouting. They are fast and easily available. Mid to late-game, the enemy will probably have built defenses. A Mirage Tank on Hold Fire stance can provide a permanent view of an area, unless you are detected. Surveillance Sweeps are also good for scouting. As an alternative, you can use Apollos instead of dogs. They are even faste, as readily available, not limited by terrain, much more resilient, and can effectively impede a Rising Sun scout. However, they are four times as expensive as Attack Dogs.

Soviets
For the Soviets, War Bears will most likely work just as well as Allied Attack Dogs. The Sickle may serve as a scout in areas with cliffs, because the Flea Jump can be used to jump into the cliffs, and because units on lower terrain cannot see higher levels. You can only be detected by aircraft, Burst Drones, Surveillance Sweeps, or enemy Sickles leaping onto your position. Due to their fast speed and small size, the Terror drones are effective at scouting as well.

Empire of the Rising Sun
Burst Drones — The Empire's Burst Drone is a good scout, because it can cross inaccessible terrain and is available early on.

The Empire's transforming units are also good for scouting because they can transform onto higher, inaccessible terrain.