There is this great product if you can find one. It is the Keith McMillen Batt-O-Meter. It's very small and besides testing all kinds of small batteries, it has a short cable with a 1/4" male connector to slip right into you input jack and it will test your battery just like that. Coolest thing ever. There is a PDF at this link so you can see it. It's definitely made my Fly life easier, including my Ibanez basses with no battery access boxes. I've had mine for close to 30 years. These items are extinct but with a little digging, you might be able to find one. Worth their weight in gold!
https://support.keithmcmillen.com/hc/en ... ter-Manual
