Our dog was supposedly neutered but he's showing signs of not being neutered. Help!?

Our dog Trooper was supposedly neutered at the shelter where we got him. He even has the little blue mark from the stitches after the operation; however, he still shows many signs of not being neutered (marking, aggressive behavior, mounting, and --sorry for the details-- whenever he gets "excited" it looks like he still has testicles). Is it possible he isn't neutered? Is it possible the operation didn't work? Has this ever happened before? What should we do?