From Liquipedia Starcraft 2 Wiki
Scouting is the act of revealing (either with a unit's line-of-sight or Scanner Sweep) remote areas of the map to gain information about the opposing player(s).
 General Scouting Purposes
- To uncover enemy game-play decisions
- To reveal the expansions of the opponent
- To learn the location of enemy units
- To facilitate the prediction of tech choices
- To assist game-play decisions
The act of scouting occurs before the game even begins by researching possible enemy builds. Doing this research will increase the effectiveness of scouting and improve your ability to counter the enemy. Researching the possible builds of your enemy will not only show you what he has at the moment of scouting, but what your opponent will have five minutes from then.
Look in the Strategy section of Liquipedia for each of the three races (Terran Strategy, Protoss Strategy, Zerg Strategy) and read all of the major builds the enemy is likely to perform against you. Try not only to understand the basic build order but also the unit composition and the idea behind the build itself. In most builds, you will want to scout your opponent at around 10 - 15 consumed supply - it's during this time the enemy should have made his first tech choice. Determine beforehand what the enemy should have at this point for each build. Doing so will help you understand weaknesses and possible alterations to standard build orders.
Go to GameReplays.org and check out the replays. The top replays with the highest amount of downloads likely have professional players that will play the builds located in the strategy section. Download 5 - 10 replays which contain your main race (3 from each of your possible match-ups is best). Notice the build orders each player uses. Watch how the professionals execute their build order and timings against the opponent. Actively participate in watching the replay. At every decision that the professional makes ask "Why would he do that?" In every attack, noticing the flaw each player makes is twice as important as noticing the player's success. You do learn something from success, but you will always learn much more by failure.
Visit the strategy section of TeamLiquid.net and read what other players are saying about each match-up. If your race is Zerg, for example, and you want information about ZvT, read all TvZ and ZvT threads. Find common complaints about your own race and exploit those weaknesses in your games with other people. Read about your race and how they deal with difficult counters and abilities from the enemy.
 Early Game Scouting
Scouting in the beginning of the game consists of sending out a Drone, Probe, or SCV to find your opponent's location, as well as to learn their opening build order and game plan. When scouting your opponent, you should consider the following:
- Proxy - lack of buildings in base.
- Cheese - lack of workers and more unit-producing structures.
- Double Gas - would indicate a fast tech build.
- No Gas - usually indicates a fast expand.
- Structures - the type of unit producing structures will give an indication of what build or what units the opponent will use.
- Placement - the placement of the buildings can indicate if the opponent will turtle or play aggressively.
Zerg players usually send their initial Overlords to find their opponent's base, making sure to position them close to a cliff or edge if there is a possibility of ground-to-air units. Drones can also be used to build an Extractor in the opponents base, canceling it before it finish building, continue scouting and repeat. Canceling a building results in a 25% loss of the cost. The prolonging of denying a geyser by canceling and rebuilding even up to 4 times would only result in a loss of 25 minerals and can be very worth it.
 Mid/Late Game Scouting
As the game progresses, higher-tech scouting units come into play.
- After creating a Barracks, Terran can upgrade their Command Center into an Orbital Command which can cast Scanner Sweep on a portion of the map, revealing any enemy units and buildings. That, however, is often not the best choice because Orbital Command energy can be used for MULEs instead.
- Terran players can also float their buildings for scouting. Their low speed is compensated by a large amount of hitpoints for relatively low cost. A well-executed example of this tactic is employed by TLO in TLO vs Nony on Lost Temple. The Barracks is comparable to the cost of a Scanner Sweep ( 150 + SCV time off the mineral line, vs. 270 from a MULE over the next 90 seconds), but has the advantage of being able to linger over the base by moving to the fringes of the map and brought back again for a peek when ranged defenders are away. One disadvantage to this is that you give your opponent information about your build. For example, if you are going for a Barracks / Starport heavy build and use your floated Factory to scout, you are telling your opponent that you required a Factory for something, but do not plan to use it.
- When Lair tech is completed, a Zerg player can upgrade Overlord speed, enabling Overlords to fly quickly into an enemy base and reveal a glimpse of that enemy's current buildings and tech.
- After the Lair tech, Overlords can also be upgraded to Overseers, who can use the Spawn Changeling ability to walk a unit through an opponent's base or to follow a hostile army.
- Burrowed units can also be used to scout.
- The Protoss detection unit, the Observer, is most commonly used for scouting in the mid game. Once Gravitic Booster has been researched it is a small and agile unit. The first goal is to get an Observer into your opponent's main base, to scout their army composition. After that, or if you're blocked, spread the Observers throughout the map to scout for army movements. One patrolling Observer is a good way to scout for hidden expansions.
- With the Sentry upgrade Hallucination, you can summon a Phoenix or an other high mobility unit do scouting for the duration of the spell. In Wings of Liberty, this is a cheap (100 ) and safe way to scout your opponent's base without having to worry about losing a unit. However, if you do not get a Robotics Facility any time soon, Hallucination would be cheaper and faster than getting a Robotics Facility and an Observer. There is also the benefit of potentially deceiving your opponent of a false tech route. In Heart of the Swarm, Hallucination no longer requires research.
- Another option for mid to late game scouting is the Dark Templar. Even though it should first and foremost be viewed as an offensive unit for the Protoss, if it is blocked by an Overseer, Missile Turret or Observer it can be left outside the opponent's choke to catch lone SCVs, Probes or Drones and scout army movements. It should also be mentioned that this is a far less economical choice to make.