How do you know if a man truly loves you?

I hear so many stories about men who appear devoted to their wives, so loving and attentive then they have an affair. Not dissing men just saying. This has happened to several people I know in family and friends and it scare me. My man is lovely but I sometimes worry that he might just up and leave one day saying he's met someone else. He says he loves me and shows it by trying to please me. But how can us girls be sure?