Sometimes size...but leg bands are the only sure way. All mine look so much alike I can't tell any of them apart except the one that keeps a slightly poopy butt. I have put different colored zip ties on mine. On the RIR's they are on the right leg, the BO's on the left leg.
for me it's the feather patterns around the neck, the sounds they make and the facial structure. It takes a second sometimes but I generally know who's who.