When contemplating entering a home you think might have zombies in it, a good rule of thumb is to knock on the windows and doors and see if you can stir up some activity. Ghouls tend to go dormant if they aren't chasing something or eating something, and nothing is around to distract them. Don't forget the second story windows, either. Toss rocks up there or something.
The important thing is to be patient. Wait for the zombies to come to the windows or doors, or see if you can see movement in the house. If there is a lot of activity, trust me, its not worth you life to go in there just because you feel like busting on some dead.
Houses have a lot of corners and angles and places for zombies to hide. Check the rooms thoroughly, and that means even looking under beds and couches. Who knows if a little one chased the cat under the bed for a snack?
When you have thoroughly cleared a room, move carefully towards the next one. Never leave an unchecked room behind you if you can help it.
As always, be aware of everything. If something is out of place or doesn't feel right, get out. Trust your instincts.
Last, don't look for supplies until you have cleared the house, and don't spend more than ten minutes in a house if you can help it. That's generally how long it takes for neighboring ghouls to surround you.