How do I check for somthing draining my battery

I was just wondering how someone would figure out what is draining their battery, I woke up today and my battery was just completely dead only the
lights on the dash would lite up a little, put in another battery and went and tested the old battery, its good, also I tested the alternator and its working too. Any suggestions? This is happening to my 94 dodge spirit with 94k miles.
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