Scouting is a tactic that is used to gain intelligence on an enemy, including troop movements, potential targets, finding weak points in a 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 quick, fast, and easily availible. Mid-late-game, the enemy will probably have built defenses. A Mirage Tank on Hold Fire stance can provide a pernament view of an area, unless you are detected. Surveilance Sweeps are also good for scouting.
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, Surveilance Sweeps, or enemy Sickles leaping onto your position.
==Empire of the Rising Sun
The Empire's Burst Drone is a good scout, because it can cross inaccessible terrain and is availible early on. The Empire's transforming units are also good for scouting because they can transform onto higher, inaccessible terrain.