[Doc]

very hard to say exactly. He has called $1200 into a $2400 pot, but he'll be out of position for the hand. I'd say he has to have something decent to do that - unless the game has been hyper agressive.

The flop check says that he is either drawing or very confident in his hand. On the face of it, it isn't a particularly threatening flop for many hands that might have raised him. He's called a 2/3 pot bet though, so he definitely has something. A nine is a possibility, but with two diamonds he might well lead or check raise here. I'd say more likely he is slow-playing a big pair or something like 88 or 77.

The turn check is called so we don't get much extra information there

The river is interesting. His opponent has made a 3/4 pot bet after checking the turn, when he could have checked it for free. Having raised pre-flop and continued to get money in the pot you have to read him for part of the flop. I can't see most players check-raising all-in here on a bluff. The A or the 7 has helped him I feel.

A9, A7, 77, 99 and AA are all possibilities here. I'm going to go for AA.