آپ know when آپ look at them and get weak in the legs and that آپ can't breath when they smile at آپ یا (sorry for saying a twilight line) it's no longer gravitey holding آپ to earth it's them. آپ think about him/her all the time and you'll know.
آپ don't know, really. آپ just have to trust yourself.
I don't think it's about shortness of breath یا weak knees یا anything. I think it's about understanding and the willingness to put up with their flaws, no matter how severe. If you're hoping for the person to change in one way یا another, then it's probably not the right person.