NL25: KK - Good preflop fold? Just kidding What is the river decision?

[ranka]

PokerStars No-Limit Hold'em, $0.25 BB (6 handed) Poker Stars Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

Reads:
Villain is CO and totally unknown this point.

Stacks:
MP ($28.95)
CO ($26.40)
Hero ($30.70)
SB ($48.30)
BB ($4.30)
UTG ($5)

Preflop: Hero is Button with K[img]/images/graemlins/diamond.gif[/img], K[img]/images/graemlins/heart.gif[/img]. MP posts a blind of $0.35.
<font color="#666666">1 fold</font>, MP (poster) checks, CO calls $0.25, <font color="#CC3333">Hero raises to $1.75</font>, <font color="#666666">2 folds</font>, MP folds, CO calls $1.50.

Flop: ($4.20) Q[img]/images/graemlins/diamond.gif[/img], 2[img]/images/graemlins/spade.gif[/img], T[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
CO checks, <font color="#CC3333">Hero bets $3</font>, CO calls $3.

Turn: ($10.20) 8[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
CO checks, <font color="#CC3333">Hero bets $6.5</font>, CO calls $6.50.

River: ($23.20) 9[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
CO checks, Hero ?

Sorry, this is maybe so standard but I ask anyway. I don't know which one is better, sometimes I lost too big bots by betting this river or lost value by checking. What is standard line here? I assume check? I put villain on flush draw or weak queen.