There's a whole ton of bluster in the torrent community. Bittorrent is a simple protocol, to the point of being naive (and therefore a fun toy network app project). Clients identify themselves to the tracker by user agent; there's really nothing else to fingerprint against. Claims to the contrary are almost certainly bullshit to scare people out of editing their user agent.

Clients also self-report the amount of data transferred. That's not great in a community that fetishizes share ratios. I've heard an op say "there's no excuse for having a ratio less than 1", which makes as much mathematical sense as the parent who told my (math) teacher friend "this is [private school], no student should be below average".

You can theoretically verify upload/download numbers because the total amount uploaded in a swarm should equal the total amount downloaded, but there are all kinds of reasons why the numbers wouldn't match. Maybe a client lost connection and couldn't send its final announce. Maybe one client is sending bad data (I'm not sure how that is reported, might be implementation specific). And clients only send transfer total updates when they connect to the tracker to change status or request more peers, so every client will have a different degree of staleness.

Even if you can tell that someone in a swarm is lying, who is your culprit? As long as they're not being egregious, there's no way to tell.