If I use 50mA of it how many hours will it last? Is there someway I can check how long is it a battery last?

It's very important to understand the difference between A/mA (ampere/milliampere) and Ah/mAh (ampere-hours and milliampere-hours).

Ampere is the rate at which the electrons travel through the conductor. One Ampere means about 6 Quintillion electrons per second. If you keep that up for one hour, one ampere-hour of charge has travelled through the conductor.

So if a battery has 10 Ah (ampere-hours) of charge and you draw 1 A (ampere), you get 10 hours of usage. And if you draw 2A, you only get 5 hours of usage.

In your example, you're drawing 50mA of current. If your battery has 550mAh of charge, you just divide 550mAh by 50mA and you get 11h (hours).

If I get a battery that I dont know if it was used, is it possible to check how many amps it still has available?

No. For some battery types you can get a rough estimate by looking at the voltage (under load), this is what battery testers do. But there are some types like Ni-MH where the voltage really doesn't say very much about the state of charge.