I wouldn't know from experience. I ignored the disfellowshipping offenses I knew about. That included people having sex, people getting drunk, people smoking, people gambling (I think they can DF you for gambling, right?).
But there were plenty of snitches. There was one sister who always went to the elders with rumors and suspicions about me. If the elders couldn't prove anything, she'd see that everybody else knew anyway.
I would guess most disfellowshippings are due to a snitch. Although there are those people who feel guilty and feel the need to confess, I would guess that's not the majority of them. Or if they do confess, it's because somebody caught them and is giving them the opportunity to turn themselves in so it'll go easier on them.