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.