3. Often there is a weight difference at birth which persists for some time.
That’ll get you through the nugget phase without an accidental swap. I have no idea what you do once they are basically the same size and start actively working together, though.
4. When they were first born, my husband intently studied them, looking for any difference. They were .25″ and 1 oz apart at birth, so we didn’t have the luxury of a weight discordance. Twin B had folded cartilage on his left ear due to his location in-utero. Once that subsided, Twin B’s stork bite became inflamed, while Twin A’s did not. That got us through until about 18 months. Now, they have slightly different head shapes, but Twin B has a freckle under his right ankle.
Also, from the beginning, we always kept Twin A on the left and Twin B on the right and assigned them colors. It’s the only way I know who is who in pictures.