I almost always lead out with my sets. One of the rewards of playing really aggressively is that when you make big hands like flopping a set, you get paid off much more with them. This is especially nice when someone holds Ax and an ace comes out with your set-card. The only exception is when i know (or am almost certain) i'll get the opportunity to check-raise, from someone who has position on me who raised pre flop and i have seen them represent the flop almost every other time they have come in with a PFR.

I don't think i ever really put someone on a set with any degree of certainty. At least not online anyway. Almost definitely not enough to lay down something like top two pair. If you make a big hand like top two, and someone has you on 4 outs cuz they made a set...you're probably paying them off. At the limits i play at least, a set looks like just about anything else. Could be TPTK for all I know, most of the time. Especially if it's an aggressive lpayer.