Villain is 30/26/22 over 92 hands (last number is 3bet percentage)
Full Tilt No-Limit Hold'em, $0.10 BB (7 handed) - Full-Tilt Converter Tool from FlopTurnRiver.com

CO ($3.36)
Hero (Button) ($10.07)
SB ($2)
BB ($15.88)
UTG ($2.36)
MP1 ($10)
MP2 ($10)

Preflop: Hero is Button with K, A
4 folds, Hero bets $0.30, 1 fold, BB raises to $0.90, Hero calls $0.60

Flop: ($1.85) 7, 7, 3 (2 players)
BB bets $1, Hero calls $1

Turn: ($3.85) 9 (2 players)
BB bets $2, Hero folds

Total pot: $3.85 | Rake: $0.25

So basically I flat preflop because I don't want to chase out worse hands.
After he bets on the flop I put him on an overpair or he could be doing this with two overs if he feels the flop has not hit me. I am in this situation a lot with two overs facing a half-2/3 pot bet and I always feel as if the opponent is betting with air.
I call on the flop and I fold the turn

The reason i am posting this is I am having difficulty being able to distinguish when my opponent is bluffing here and when he isnt (on the flop). He three bet pre flop and I guess he sensed weakness in my flat call preflop and could potentially be betting air here.
but how would I know?

Thanks