Here is the adorably cute article about Welsh triplets, whose parents paint their toenails to tell them apart, that inspired this question.
When you look through old family photos and such, can you tell yourself from your sibling (assuming you’re wearing the same clothes)?
We can tell each other apart in most of our baby pictures. I had weak neck muscles as an infant so my head was often tilted to the side. Such flicktedness helps with ID’ing. 
But there are a few pictures of us where we are indistinguishable. We actually destroyed one such picture when we were teenagers, while fighting over which one of us was the “cuter” baby in the foreground. There’s also a photo of us taken when we were three. It’s a close-up picture, and everyone really should be able to tell with 100% certainty who’s who. I have that certainty. But my sister doesn’t.
Strange? logical? but she’s the one who looks familiar to me.
The Dionne quintuplets could (or claimed to) always be able to identify which was which in the old photos and films of themselves. Not just pick out themselves, but identify each one in the photo.
FWIW, the one who had epilepsy and died at age 20 is easy to pick out in photos when they are in their teens, because she looks heavier-- it’s a side effect of anti-seizure medications of the time. Supposedly, also, one of them was always taller than the others, but I don’t know which one.
Not a twin, but much younger sister of identical twins. So I never knew them as children.
I rarely doubt which is which. Since I know the minor distinctions between their appearances as adults, it is easy to detect those differences in their appearances as children.