Here’re my guidelines for second barreling:

-If you pick up some outs on the turn, then be more likely to fire a second barrel. If you think your villain is strong and you are in position, check the turn and try to catch your card and bust him, instead of betting the turn and risking being raised off.

-If a weak player calls on the flop when its draw heavy and no obvious draws complete on the turn, then be more likely to fire again. In this spot be even more likely to fire again if your hand has showdown value (like AK or a small pair).

-If a solid player calls you on a paired rag board like 779, be more likely to fire a second barrel.

-There’s no standard situation to second barrel. Don’t overuse this play.