80% acccuracy and 5.11 shots per kill. he'd be aeons ahead of everyone else then choke on knife because he's left clicking. So either he's left clicking to stay on knife cause he knows he's being watched and is hoping I'll leave. Or he's a noob with hacks that doesn't know how to knife. Either way, his accuracy and shots per kill give it away.
I have a demo of half a game but he was on knife the whole time. Not sure if that will really help you.