To check the battery just do a simple load test on it at your local garage they can tell you what shape it is in . next thing you need to do is check for a dead short in the system , i do that a quick way by disconnecting the positive wire and scratching it on the + terminal on the battery if it makes sparks then power is drawing if not then it's a bad battery .. good luck . all the guys tell you oh go buy a power tester and put between the battery and the wire bla bla bla im tryin to get people the fastest and cheapest way to find the root of the evil. cars nickel and dime a person bad enough dont waste money on a tester to see if you have a draw just do the scratch test .. believe me it works . all you guru's want to sound all high and mighty lol use a tester ...
now if you do have a very small spark it can be a clock drawing power if it is a major draw it will make a snaping loud sparking sound and you'll see the spark jump from battery to the cable . p.s dont try this on computer controled cars / the computers are to sensitive and you could fry it, thats when the testers come in handy .