There is no way that you can tell if another user is sharing files. For instance they may upload on a DSL connection on one computer and download on a wireless connection on another one. In short you cannot tell, but you get a feel for what they are doing if you study their upload patterns. If I get a n idiot queuing 50 or a hundred songs from all over my different shared folders, I will block him without hesitation. I doubt very much if he would be sharing.
Your codec thought souns correct. I'm not sure myself what you need, but google might give you the answer.
I am wary of files shared by multiple hosts. It is often these are empty files, advertisements for iPods or viruses. Most of my files come from single hosts, but if there is a small number of hosts with a file I want, I will give it a try. The file sizzes are a helpful giveaway. Always check the file sizes of what you are down loading, and do not confuse Kb with Mb.
I hope this helps a bit ... I'm tired and rambling ... so I'll get some sleep
