Three hours at the gym sounds fishy to me too, not to mention working overtime the same amount of time. If he is cheating then nagging him with circumstantial evidence will not make him stop. It will just make him try harder to avoid detection. My advice is act like you believe him and hire a private investigator to tail him. You want him to have his guard down, he will be easier to catch that way. If it turns out you’re right get as far away from the bum as possible. Until then you don’t have enough evidence to be making any accusations.