Reading Players Online

I recently won a SNG at Full Tilt and I was presented with a hand that required a little reading of the other player.

We’re in the third level of the SNG and I'm the chip leader. 

I'm Under-The-Gun (first to act) preflop and have Ad-Qc. 
As chip leader, I decide to lead out and bet four times the BB to 200 to see if I can win this pot now.

Seat 4 and 5 fold.
Seat 6 calls
Everyone else folds.

The flop comes 4s-Jd-4d.

I'm guessing this flop didn't hit either of us.  But I don't want to lose a lot of money.

I check and seat 6 checks.

I'm pretty that flop helped no one.

The turn comes a Kh.

Since I'm in early position, I want to see what my opponent does.

I check.
Seat 6 bets half the pot at 250.

The only question I have at this point is "does my opponent have a King?"

My gut tells me that he doesn't based on his current chip stack which is at 1000 at this point.  The reason is that the half pot bet feels like a probe bet.  If he had the king at this stage in the tournament, he'd be all-in.

I call.

The river is 3h.  I'm 100% sure he doesn't have the 3.

I check.
Seat 6 goes all-in.

I think for a second.  With Ace-Queen, I have the high cards and I have the best hand.

I call.

Seat 6 shows 10d-9d. He's on a flush draw.

I win the pot and knock out player 6.  Another player at this point chats "UR kidding!"

I respond "I knew he didn't have it."  Come on, I would never have gotten that deep with a player if I thought he had a hand.