The better you know the game, the more you will pick up from clues about what the opponent is doing based on the amount of time that has passed and what he would be able do have done in that time.
For example, if you scout the enemy base, and you know that a Terran opponent should have been able to make at least three barracks by the 4 minute mark, and only has one, chances are good that he has a hidden expansion somewhere.
I recommend sending a scout to the enemy base when you've hit 9 or 10 supply. That is, you'll have 9 or 10 workers at this time. The reason you send one so early is that some early strategies, such as the 6-pool, will require you to change your build order quite early to defend against the early attack. Furthermore, your opponent can wall-off his base such that your worker scout cannot enter the base, thus denying you ANY scouting information.
You should check the following things with your early scout:
- Gas, present or absent
- Number and/or timing of Barracks, Gateways, or Spawning Pool
- Number of Workers
No gas for Terran and Protoss usually means an early attack or expansion, because they are forgoing tech in exchange for a fast expand or a low-tech rush.
The number of workers is also important, because early in the game, it is difficult to afford both workers and a constant production of attack units to offset the defender's advantage.