Slowplay on the river question

[adam5206]

This isn't a specific hand, but its came up alot in the last 10 or so games.

Say you and one other player see the flop and ur behind him. He bets, you have top set with no draws on board so you decide to slowplay and just call. Useless card on the turn u still have top set with no draws so he bets u call. Safe card again on the river and u have the nuts. He checks, what do u do now?