It's easy to tell the difference between my two - Franklin weighs about twice as much as Chelsea.....
When in bed I can also tell what kind of mood they are in without actually seeing them. Chelsea will gradually tense up before she whacks at Franklin (sometimes a momcat need some alone time!) and Franklin has a certain "feel" when he is about to drop some toy on my chest, wanting to play.
Nothing special - just the stuff any cat person knows about their purrkids...