In the 50th anniversary episode when the 10th Doctor saw a time rip, a person came out of it then this conversation happens:
ELIZABETH: Who is this man?
DOCTOR 10: That's just what I was wondering.
DOCTOR: Oh, that is skinny. That is proper skinny. I've never seen it from the outside. It's like a special effect. Oi!
DOCTOR: Ha! Matchstick man.
Then the 10th suddenly looks at him like he recognizes him, then they say this:
DOCTOR 10: You're not. (They both get out their sonic screwdrivers. 11's is bigger and better.)
DOCTOR 10: Compensating.
DOCTOR: For what?
DOCTOR 10: Regeneration. It's a lottery.
DOCTOR: Oh, he's cool. Isn't he cool? I'm the Doctor and I'm all cool. Oops, I'm wearing sandshoes.
Which made me think: how did the 10th Doctor recognize the 11th as his future self? He didn’t regenerate into him yet, so how did he know he is his future self?