Wow, a lot of players here think bet-sizing (particularly on the river) is a good way to tell if a bet is a
bluff. This might explain my great success with over-betting the pot with the nuts on the end, since so many people here think big bets mean weakness. I am sizing my bets, on the end, based on many things but a lot of it is based on my perception of the strength of your hand. If I expect you to call, and expect to have the better hand, I am betting larger because I want more money when you call. If I suspect you have a weak hand, one that can't call a large bet, I bet smaller to get a call and make money that way.
Now, when I am bluffing on the end, the same rules apply. If you are probably weak (or your range is highly polarized to air or the nuts), I size my bluffs smaller. I can save money when I get raised or called, but I still expect to win. When your range is strong, but likely to have gotten worse as the board came out (turn and river hurt your range), I am likely to bluff more on the large side, because I expect you to find a way to justify paying me off with a good hand. Actually, I often will pass on these spots unless I know you can fold there, because most micro players find a way to justify calling even large bets.
I think, for many players, bet sizing is not a good way to detect bluffs. Many players bet large because they have a big hand. They're not thinking about what you have, they're not thinking of bluffing. They're just thinking that they have a straight and straights are good.
A bluff is something I pick up when a bet doesn't make sense or fit the previous play of the hand and how the board has developed. You play enough hands, and you get a feel for how a hand is going to go depending on which cards come. You also have an assumed range of hands that someone will have. When they suddenly come to life and make a bet that's unusual and doesn't fit their range of hands, it causes me to stop and think a bit harder. Did I misread their range? Did they decide to get tricky or play unusually earlier in the hand? Does what their bet says they have make sense?
Depending on how I feel about the answers to those sorts of questions, and where my own hand falls in the range of hands I have in that spot, I will often make a call. I don't make the call because I "know" they're bluffing this time, sometimes they're not. I make they call because I expect them to be bluffing often enough that it's profitable. I trust my feelings, which are based on lots of experience and being in similar spots.