I haven't been playing online lately so I'm just going to make up a situation.

FR .05/.10 NL

BTN: Hero
UTG: Villain

UTG raises to 3bb
Folds to Hero
Hero calls 3bb w/ JdTd
SB & BB fold
Flop (8bb): 6d6h2d

Villain checks, Hero checks

Turn (8bb): 8c

Villain checks, Hero checks

River (8bb): 9s

Villain checks, Hero bets 6bb, Villain ???

With no reads on Villain at .10NL I think it's important to allow that some non-standard hands for an UTG raise are in the PF range. Basically my intuition is that hands like AJo, KJ and AT are active here sometimes.

So say he opens {77+, AJ+, KQ, 6 combos of KJ, 6 combos of AT}
His post flop behavior pretty much rules out 77+ and suited diamonds. I'll leave AJdd, AQdd, and AKdd to allow that he doesn't always cbet the flush draw. We'll leave one combo of AA and one combo of 77 to represent the chance he's the passive sort or the sort to play high PP's super slow. I'll also pretend he cbets 50% of everything else.

This leaves Villain on the river with 1 AA, 1 77, 6 AJ, 8 AQ, 8 AK, 7 KQ, 3 KJ, 3 AT for a total of 37 hands. I'll say Villain calls with all of the A-high hands half the time, AA, 77 (or whatever other pocket pair he may have not bet postflop with for whatever reason), and folds the rest. This means he's only calling with (1+1+3+4+4)/37 hands and 13/37 is <<< than 6/14 which makes it a pretty profitable bluff.