Would you bet this turn?

[ScottHogfoss]

In a 5 way game on Pacific with 2 bad players. One of them raises UTG and I call in next position with 68h. Button folds. Other bad player is in SB. He calls. BB calls too.

Flop: A 8 5, two clubs.
UTG bets, I raise, SB cold calls, BB folds, UTG folds.
Turn: 4 spades.
SB checks, I check.

CAN I BET HERE?

River: K hearts.
SB bets, I call.
SB has J9 clubs, I win with pair of 8.

How do I know SB has flush draw and not a good Ace or better? Do I have to bet the turn and find out?

GWH