Even though there was no official test of strength between Zeus and Poseidon, from the Olympians' plan to dethrone Zeus and didn't prevail, Zeus showed his power over Poseidon by sentencing him to hard labour, where he was forced to help build the walls of Troy. If Poseidon was indeed the most powerful, he could have launched a coup against Zeus all by himself to show he wasn't under any command or rulership of Zeus.