A couple months ago I was having modem problems and would get kicked off and might not get a connection for 10-12 hours. It's very frustrating (particularly when you're ahead).
I just quit playing multi-person games until the cable company fixed it.
Sometimes you can tell, though. I was playing a guy, had him down 4 after 4 holes. He just disconnected. I'm pretty sure it wasn't an accident. At the very least, one can bring up the menu and concede the match, if it's gotten out of hand. I mean, we all make bad shots from time to time, and sometimes one right after another.
Best to just be courteous, tell the opponent you're going to concede the match, and move right along.
There seems to be an epidemic of rudeness these days, though. Pity.