Scouting without knowing what you're looking for is sightseeing. Before you send a unit, ask yourself one question: did they expand? how much gas did they take? where is their army?
What to look at
- Bases: how many and when they were taken.
- Gas: lots of early gas often means tech or an attack.
- Production buildings: how many and which kind.
What to do with the answer
If what you saw doesn't change your plan, stick to your plan. If it does, decide fast and commit. The Information before reacting routine helps you practice it.