how do you know if you are truly over someone?

i want to be fully over my ex before i get involved with anyone else. but since he’s the only guy i’ve ever been with, i’m curious as to what that actually means. can you ever really get rid of all feelings? do you get to a point where it wouldn’t even bother you to see them with someone else? i wouldn’t want to date my ex ever again, but i still feel sad when i think about him. how should i feel and view him once i start to date again?