I think you just know. For most people it's way before you start to hate each other, with my first wife I just found everything about her suddenly irritating and ridiculous and used to relish any time I could get away form her. Thankfully she felt the same and divorced me.
When i avoided sharing the same bed, when him even accidently touching me made me want to spew! No conversation and when i was tempted to cheat..(i didnt) xx