You could always look up the lawfirm, call them see if its real etc. Almost always though, the letter will come from the ISP (if not always) in which case they will give you details, your IP, what you downloaded from where using what. (I've dealt with about 5-6 myself because my idiot friend downloaded crap without encrypting!) Anyway its pretty much a 75% chance fake due to it not coming from an ISP, as most big companies "usually" won't do such a thing (I say usually but it can happen in some rare off chance).