what are some clues that lead percy and the campers to realize that he is the son of poseidon the god of the sea

They run into Clarisse, a daughter of Ares. Clarisse sneers at him and drags him off to the bathroom, where she tries to push his head towards the toilet bowl. Suddenly something happens, and the toilet shoots water out right at Clarisse. Percy cannot figure out how he did it, but he uses his newfound skill as a threat to make sure Clarisse does not bother him again. Annabeth tells him she wants him on her team for capture the flag.