Add your methods or the ones that are not mentioned, I am going to list mine.
1) Send lonely lings, marines, zelots to just run in to enemy bases and armies so that I would know where they are.
2) Send observers to scout enemy bases, and army locations, compositions.
3) Borrow zerg lings in strategically important locations to know where the enemy is.
4) Use orbital command scan to learn about buildings and army, base location.
5) Send cloacked terran and prottos units to search for enemy.
6) Move borrowed zerg units arround the map to patrol locations and look for enemy.
7) Spread Creep all over the place to gain vision and speed.
8) Build cheap buildings like pilons, turrets or supply depots in strategically important locations to spot enemy base constructions or amy movements.
9) Send overlords around the map in key locations and hope they wont be killed off and will give valuable information on the enemy.
10) Move out and try to bait enemy in to well prepared defensive position like siaged tanks or borrowed widow mines.
11) Attack enemy base in known location with very small contingent of troops like 5 roaches to learn about their full army composition.