Know yourself, love yourself, and trust yourself.
If something doesn't feel right to you then it's probably not right for you. This question reminds me of similar ones that ask: How can you tell if your wife or husband is cheating?
It's doubtful you would jump for joy if you learned your husband was not gay but instead was having an affair with another woman. Therefore the bigger issue is not whether or not he is gay but rather or not he has lied to you and betrayed you. Whether he cheated with a man or a woman it's still cheating.
Unless one is naturally paranoid or insecure they should never ignore their feelings or suspicions about someone who is close to them. Changes in behavior is usually the first thing that sets off alarms. This may include a lack of sexual interest, becoming more distant/spending less time with you, easily annoyed with you over the slightest thing, whenever he's upset with you he jumps in his car and disappears for hours at a time and oftentimes it is he that creates the friction. Acting jumpy while on the computer whenever you walk into the room...etc
In order for someone to cheat on you they have to make time to be away from you. I once had a female co-worker who was in therapy primarily because of how her husband treated her. He made her feel undesired and constantly told her she was immature among other things. She decided to get help in hopes of becoming the kind of woman he would once again adore. One day during lunch she received a call from the police station. Her husband was arrested for having sex with another man in the restroom of the mall. Naturally this tore her up and she filed for divorce over his protest. His parents didn't know he was gay.
The truth is she was (unhappy) in her marriage long before she found out about his sexuality and the cheating. She got tested for HIV and thankfully it came back negative. Bottom line it shouldn't take cheating or some other major discovery to give yourself permission to walk away from a toxic or unhappy marriage.
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde