First, find out if your seller is an authorized dealer. If not from an authorized dealer, compare the exterior of your headphones very closely to pictures of m50's online. Then if you're still unsure, take it apart and compare it to pictures of the insides of genuine m50's. I found no reference to counterfeit m50's after a brief google search, coupled with the fact that you think it sounds good, it's unlikely you have fake m50's. There are certainly fake esw9's. Audio-technica and sony have many of their headphones counterfeited.