In most amplifiers there will be two output transistors and if it's stereo they will be replicated in the other channel. Can you match the numbers you can see with the other ones?
If that isn't possible, with the failed transistor removed, power it up and measure the voltage between the collector and emitter connections, if the collector is more positive it will be NPN (MJE3055) otherwise it will be PNP (MJE2955). It would be wise to replace both output transistors, if one is shorted, the chances are the other has been overstressed. Those types are not expensive.
Brian.