Diagnose bad brake cylinder?

It will either leak or it will be siezed in the bore. Leaks will be obvious. Sieizing will cause pulling to one side or dragging brakes and the drum may be very difficult to remove. You may see excessive wear on one shoe caused by failure to release.

You will see brake fluid leaking, and your pedal will be spongy

Hi so how do you know what it is this takes years of knowledge.

the cylinder will leak, or it will not release

They will leak. If they're not leaking, there is air in the system and they need to be bled.