how to determin max current for an unknown stepper motor?
As @Andy aka said, If you want to be professional, buy new ones.
If you still want to try and use the ones you have, you can check the resistance on the windings, note the size of the stepper, to have a reference of the resistance there, and check for similar size stepper motor's datasheets.
If you manage to find similar datasheet of the motor you already have, and their internal resistance is the same/similar to yours, just use the same current and voltage rating that the datasheet suggests.
Operate your stepper on this suggested current and check its temperature. If it is cold while you operate it, you can increase the current. Steppers can get hot, so hot that you cant touch them.
You can also check similar datasheet's maximum temperature