Depends on what you are "bench testing". If you are "benching" it just to verify that it works, then you can do as described above.
If you are benching it to determine the amplifier's actual output, then no, you generally can't do it yourself as it would require components that most people don't have access to such as an oscilloscope, large 12V power supply (much larger than 20A....100A+ depending on what size amplifiers you are benching), large dummy load, etc etc. Best to just find a local shop that already has all of the necessary components to do it for you.