easiest way is to get a test light. unhook one of the battery cables and hook the test light between the post and the cable. if light lights up nice and bright you have a battery drain. make sure everything including dome lights are off when you hook light up. if it is bright you can start unplugging fuses and relays in the fuse box under the hood until the light goes out. once you know what circuit the drain is on you will know where to look