SB is your typical fish while BTN is a OK reg running at something like 17/14. I'm pretty sure that he is raising here with set or a good draw while I'm almost positive that he doesn't make big time bluffs on turn.
$3/$6 No Limit Holdem
5 players
Converted at

Stacks:
UTG ($822.50)
Hero ($600.00)
BTN ($639.15)
SB ($423.00)
BB ($616.05)

Pre-flop: ($9, 5 players) Hero is CO [Kd]
1 fold, Hero raises to $21, BTN calls $21, SB calls $18, 1 fold
At first I thought that BTN might put me on a decent hand and he decided to call with his draw. He could also have Tx type of hand that is uncertain where he stands.

Flop: [8c][4d] ($69, 3 players)
SB checks, Hero bets $50, BTN calls $50, SB calls $50

Turn: ($219, 3 players)
SB checks, Hero checks, BTN bets $175, SB folds, Hero ?
At this point, only 2 hands that beat me are 76s which is unlikely to see turn unless 76c but he would push that, a floating 99 or T9. However, he is not making moves on turn so this is rarely a bluff.
What should hero do ?