Perhaps if he starts looking after himself more then usual, protective over his privacy on his phone ect, works more often then usual and it seems like he is always in another place in his head, like being disinterested in you.
If you think his cheating, the trust obviously isn't there in the first place. This may not be your fault, especially if he has done something in the past. Therefore, whether he is cheating or not, maybe the relationship isn't working in the first place to believe he is. Nobody deserves to have to worry about things like this, you should find someone else and be happy knowing you can trust them