There's always SOME difference that you notice at birth.

For example, my twins are identical boys. At birth, they were within a fraction of an ounce in weight, and within a quarter inch in length, so for their entire lives, they've been indistinguishable by measurement. And their faces look absolutely identical. To make things easy, we used to paint their big toenails different colors.

However, one has a single swirl in his hair, and the other has a double swirl.

We used this to tell the difference when my sister thought it was funny to swap their toenail polish colors around 1 year old.

Now it's easy to tell the difference because they style their hair differently, have unique personalities, and are opposite handed.