Did Paulie know he was about to get whacked when he went with Rocco and Clemenza to check out some "gooda spots on the west side".

I think he did. His suspicious look and the way he said, "Hey Rocco, sit on the other side. You block the rear view mirror." Show that he knew something was up.

Plus, after the failed hit on Vito, Paulie must have known he was a prime suspect. Note the way Sonny was being overly nice to him. Paulie must have felt that he was under scrutiny.