Similar to what Angelic_Ace said, I would suggest that you bet out on the flop to quantify your position. You know nothing of his holdings after a check.

If he is drawing to a straight, give him improper pot odds and let him call. When you hold K 7 and the board comes K 7 2 with a rainbow you can slowplay with confidence. But just take that pot down if it looks too dangerous to give free cards.

If, as in this case, he already has the straight... well he's going to come back at you with a reraise on the flop or on 4th, and you'll be getting the signals that you're beat and need to lay it down.

But I wouldn't listen to the signals in that situation either.. =) two pair is just too tempting.