I stole this from another forum................
You probably already know this: you can verify if you have a drain with an voltmeter (in ammeter mode) (you can get this for $10 in wal mart)
Disconnect negative wire from the battery and attach it to positive post on voltmeter. Attach negative volmeter's post to
negative post of the batter. With this setup, any current that's flowing through the car circuits will show up in the ammeter.
On a normal car current should 0 or negligble (probably radio will draw tiny current, etc).
So, once you set this up and (hopefully) find that you have current draw, start poping out fuses one by one and watch for current to go down.
btw, don't try to crank the car with this setup, you'll fry your ammeter.