How do I make sure I don't mix up identical twin newborns?

I'll assume that you're not just being facetious here. As the father of new twins, I can tell you that just keeping track of the state of a single baby is hard, it's more than doubly hard with twins. Has Joe eaten yet? What boob is Bob on? Have you changed Tom's diaper yet? Does Ben need to burp still? While each of these questions has very little long term consequence, each one can have very serious short term psychological impacts on the parents. When you're already over stressed and sleep deprived, missing something as simple as a diaper change can cause a catastrophic failure mode (aka crying) that interrupts the one rem cycle you were gonna have that night.

So, keeping track of which baby is which ends up being essential for mental health.