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Chase will find out near the end of the main story, like when ~6/7 of the story it done. And it's not that there were not enough clues there before, because those clues were there, but Chase was oblivious, ignored them and believed Josh's excuses, kind of all of it.^^
When he found out there was no other option than that Josh was an omega.^^"
Most other alphas would have known much earlier with the information Chase had.lol

I forgot to mention, that is only when he finds out that Josh is an omega. If you also wanted to know when he finds out that Josh is the omega slept with back then, that happens in the last chapter of the main story. Let's say Chase is not the sharpest when it comes to Josh and needs everything more of less spelled out for him.lol
When will chase know that josh is his omega or that he is an omega in the novel?