How do you ask your boyfriend if he is cheating?

Start by asking him if something is bothering him. Tell him you've noticed some changes in him (or something like that). Ask if he's still happy with you or if you are doing something wrong (this makes it look like you think its you, not him). Flow withh the conversation after that. Be prepared if he is cheating though.

Don't accuse him right away. That will just get him mad.