I’m Not Complaining! — What Are the Indicators for a Bad Battery?

I’m not here to notoriously complain about bad battery drainage, defend good battery capacity, or whatever.

I was just wondering where I can find a good clear and concise list of things to keep an eye out for to indicate a bad battery that would warrant getting it replaced or checked by Apple.

What to look at, tests that I can do, numbers, etc. that I can apply to multiple phones.

I realize batteries vary, but just even a general margin would be helpful.

Thanks.