How to differentiate kicks

My doctor said I should be noting each baby's activity and asked if I can differentiate their kicks. I can't really, except by guessing where they are positioned. Problem is they change positions quite often. I was a little annoyed too because she made it sound really serious that I make sure they each kick about 10 times, but didn't really explain much besides that. They should each give 10 kicks around the same time, at any given time? And how do I accurately differentiate which baby is which? She said if their activity suddenly lessens or changes, that I should come in to get an ultrasound, so now I'm confused and paranoid... Can anybody relate?