How should I talk to him about something that's been really bothering me?

Okay, so I live with my boyfriend and we've been together for over a year. What's been bothering me id that I catch him looking through his ex's profile on Instagram and Facebook from time to time, how should I ask him about it without it making it seem like I'm accusing him of cheating on me? I trust him, but I'm really bothered by him looking through other women's profiles on social media...