How do you tell Tweedle Dee and Tweedle Dum apart?

I have a picture of me at Disneyland from 8 years ago, and I got my picture taken with one of them. However, in the picture, all you can see is Tweedle D. I can't tell if he's Dee or Dum, and I can't find my autograph book from there. (Yes, I have one, but I've moved so many times in that period of time, it's in a box hidden under a ton of other boxes and furniture.) If anyone can please help me, that would be great. In all honesty, I'm serious to find out who I had my picture taken next to.