Is my wiring 15 or 20 amp? The breaker is 20 amp

I want to buy a surge protector for my audio system. My amp got fried in a thunderstorm. The breaker in box is 20 amp, and I installed a PS audio power port, which is both 15 or 20 amp, but I dont know if the wiring is 15 or 20 amp? I am suspicious that it might be actually be 20 amp, even though it had a 15 amp receptacle, but there must be a way to tell. It seems important before I plug something in that could overload the wire which is 20 amp? Is there an instrument out there that would test it? Many of the breakers in the box are 15 amp but a few are 20 amp including the bedroom wall receptacles where I have my audio system. I know that a very competent electrician installed the breaker box 10 years ago when I moved in, because the one that was there was quite inadequate. He seemed like he knew a lot, and was from top notch company if that makes a difference.
Have an electrician check the size of all the wiring.I don't
know if you need a 20 amp in the bedroom.The electrician
should know this.If he can't trace all of the wiring to see
its size,he might change it to a 15 amp circuit to
be on the safe side.I would think this could be done only
by a visual inspection.
If it has a 20A breaker on the circuit it should have 12GA wire.If the wire is 14GA change the breaker to 15A.This is per NEC regs.You can use bigger wire on smaller breaker but not vice-versa.
All 20A breakers operate 15A circuite as per the National Electrical Code. Has to do with continuous vs. peak currents.
What Tpreaves said is correct.You could also check the wire size yourself if you feel comfortable around the panel.Shut the main switch off and use a fashlight to be able to see if the panel is in the basement.Open the door of the electrical panel and then proceed to remove the screws that attach the front panel cover to the box.Once the panel is removed you should be able to see the black wire that connects to the 20A breaker,if it's larger than the ones that connect to the 15A breakers then you know it's 12ga.
I prefer to use 10g with 20Amp breakers and 12g with 15Amp breakers for audio use.