What are some key factors in spotting bluffs. I have my own ideas, but i'd like to hear some input on how to know if someone is bluffing or not.

I'll say the thing that comes to mind right now:
Raised pot with two people, board is low with a J, first to act bets, I call thinking it may be a continuation. turn blank, first to act checks, I check. Last card blank, First to act makes big bet.

The reason this is most likely a bluff is because the first two streets he played as if he was weak and had just made a continuation. Then on the river there is a complete blank and he bets hard, his hand obviously didn't improve so i think bluff. Obviously this hand wasn't played very well by me, but thats not the point here.

So lets hear input on spotting bluffs! From LAGG players, to TAGG players