Are Athiest certain that deities dont exist, or that you can't know whether a God exists?" "How do you know that?

There can logically be no certainty that gods don't exist - you can't prove a negative - essentially you can't look everywhere in the entire universe to check that pink unicorns or tasty gnocchi or gods don't exist somewhere.

Besides which, this isn't atheism. Theism and atheism are matters of belief. Matters of knowledge of whether gods exist is gnosticism and agnosticism. The vast majority of theists and atheists are agnostic - if pushed they will admit that they can't be 100% certain that gods do or do not exist. There are a few on both sides who claim 100% - they are gnostic.

As with any question to atheists which doesn't specifically address a lack of belief in gods, the only answer is that you would have to ask them individually. There is no shared viewpoint on anything other than that lack of belief.

As for me, I am an agnostic atheist. There is no evidence for gods, no need for gods to exist and, no matter where we look, the universe works exactly as we would expect it should if they don't exist. But I can't be sure.