It's best to check their whole body, whether they have any moles big or small or special mark to identify them. So incase future mix up, and need to double cfm, can cfm by their special mark. Now can give different hair style or anklets to identify them easier.
You can check their distinguishing marks, like the size of their head, moles, freckles and birthmarks. Some parents use colour codes for their kids, like one kid use a specific colour than the other. It's fun to have twins :-) I wish I have one..or two :-)
I was babysitting my neighbors' twins (2 years) just now and I tell them apart by their hairstyles. The parents of the baby girls had one twin's hair in braids and the other one had her hair down. Too cute!
New born baby, when you call them look into their face you can see the different on their reaction.
Maybe get a different anklet for each?