Car turns over but won't fire

Hello, so, if the engine won't turn over, and it's not the battery, what are you figuring? How do you know the battery is not at least a part of the problem, right now? If you have a fully charged, strong battery, but the engine does not crank, you have a battery cabling issue or a starter motor issue. Both are really low percentage possibilities, but you've eliminated the battery. Wondering how you tested it as good?