You can do two things, if you have the right tools.

1 - Use a battery hydrometer to measure specific gravity in each cell. If they're much different from each other, that's not good. Sorta related to this, if you have to add distilled water because the levels keep dropping, that's not good.

2 - Do a load test. If you don't have a load tester, you can sorta check by measuring battery voltage with everything turned off. Then measure it again with headlights, blowers, window motors, stereos, etc... Basically turn on everything you can think of. A healthy battery won't drop much in voltage with all that stuff running.