When I had multiple cats () I could tell simply because Taz was much bigger than Paizly, or Taz would growl at Paizly. But then toward the end he actually weighed a little less than her. But he (Taz) would come up right to my face to settle in. And she (Paizly) will sleep in the curve of my stomach when I'm on my side.

I can sometimes tell the dogs apart when they jump up too. Mostly because when Taggart gets on the bed he immediately flops his head down and Zoee will either sit with her head up for a while or gently lay her head on her paws. I can also tell the dogs by the feel of their coats. Zoee's is much thicker than Tagg's.