You have a few options:
- When you build your 2nd food supply, get that scout in the base and keep it there as long as possible. You may be able to determine if he is going air.
- Watch Towers. Get a unit on a watch tower quickly. I usually use my first couple of zerglings.
- Informative 'scouting'. Scans, Sacrificial overlords, fast observers can be excellent in determining what he has and might be moving out with.
I recently had a match where because of my overlord placement and watch towers, I saw his banshees move out. Gave me plenty of time to get Overseers, hydras and an extra queen to defend.
Take a look at Map Control for more info on just that: Map Control which relates to determing what your opponent is doing.