Harvesting components from devices

When it come to harvesting transformers and motors many times I face the question of how many amps (transformer) and voltage (motors) can they handle.
in the case of transformers, I usually try to get a glance of the wire in the secondary spool and then do the guessing work. Before using them, I'll connect them and see how hot they get and if there is not much humming sounds.
Does anyone have suggestions about it? Is there a way to test them more "scientifically"??