Some posters are put on "moderation" if they have received 50 or more infraction points. It's kind of like being put on probation. Sometimes, if there are no more problems, the points fall off after a specific period and the moderation drops. You would know if you were in this category.
Some particularly volatile threads/forums are always moderated, meaning that all posts, no matter who makes them, have to be approved. One example is the CRNA forum, in which there were so many attacks and counter attacks that staff decided it was a lot less wear and tear to put an extra step into the equation. Posts that violated the TOS never saw the light of day so there were far fewer messes to clean up.
Do the moderators randomly select new posts to evaluate? Or am I in trouble?!
As I said above, you would know if you were on thin ice. As far as selecting random posts to moderate, each staff member has a number of forums to keep an eye on. We also respond to reports made by vigilant members (whom we greatly appreciate). Stick to the Terms of Service and everything should be cool.
Hope that answers your question.