Battery gauge fluctuation

Hello all, first time to post here. I have a 2007 Tundra SR5 crew cab, my baby!! Well I just put in a brand new Interstate battery, it did have one not even a year old prior. It sat for a week and then wouldn't start, was able to start with my battery jump box and it would run and I would turn it off and it wouldn't start. Got it running for a day and parked it and next day wouldn't start. Took battery to Costco and free warranty replacement, getting inspection now. The gauge does fluctuate when not driving, it will deep down under braking or at a light. It has always done that though. But I'm thinking I have a parasitic draw when it isn't being driven every day. How do you test for the draw?