Telling Twins Apart

I have twins in my class again this year….identical twin boys.  Yesterday I asked one of them what the best way to tell them apart would be.  Now I am thinking of physical characteristics.  I am looking for a telltale feature…for example, a freckle, mole, eye brow arch, a bigger nose, creased earlobe, or even thinner fingers.   I am subtly scanning his face as I wait for his response.  He excitedly takes off his glasses to show me that the inside arm of his glasses is a different color than his identical twin brother.  I might be in a little trouble here.