Go to a place that sells/services car batteries, and get them to do a "load test" on the battery, and check if its stuffed. Actually if you disconnect the battery, then unscrew the tops and you can see the cells bubbling heaps, its a good sign that its f*cked. Also get them to do a "drain test" which involves hooking a multimeter up to battery between the pos lead and the pos terminal. That will tell you how many amps your car is draining.