There's a few things you can note.
1.Damaged songs usuually have the cd name directly in mp3 file name. For example, if you wanted the song 'girls not grey' from afi's new album do not download a song with the file name ' afi - sing of sorrows - girls not grey.mp3 ' or anything like that.
2.Also, the people sharing bad songs usually are sharing about 10 different versions of the bad song under different file names. So that way if you search for a song name, alot of their versions will display. Answer: if your search gets 150 results, look at the users sharing the songs. If alot of the results are from the same user, chances are he's sharing a bad version.
3. Finally, people sharing scratched music almost always disable the 'find more files from same user' function. So, if you want to double check, try finding more files from the same user. If nothing shows up, then they could very well be sharing bad music. But if you get a huge list of thousands of songs, and the song you just downloaded works fine. Keep getting songs from that person.
Eat. Sleep. Breathe. Ski.