It's easy to tell which ones are a hoax and which are real. The average movie file size is around 700 MB. If it's less than 650 MB and in the 200-500 MB range then it's obviously a video cam someone took into the theatre so dont download those ones lol. And if it is less than 5 MB then it's a virus(in most cases). For mp3 files, it varies. But if it's less than 1 MB then it's most likely a trick too. Most of the viruses I've seen are around 130 KB or somethin. So if it's 1 MB or up, it's ok to download.

Btw, watch out for .wav files. Those ones are usually the ones that have viruses.