There are three main ways to identify if an amp is blown: you can observe physical damages, check for burning smells, or pay attention to any unusual sounds coming from it. If you spot any issues with the wires or connectors and see any smoke, flame, or burn marks anywhere on the device, then there's a good chance that your amp has become damaged beyond repair. Also, if you detect any foul odors coming out of the amp while it's running, this could be caused by an internal short circuit and should be checked as soon as possible. Lastly, listen carefully to make sure none of the speakers are producing strange noises, such as humming or buzzing, that weren’t there before.