So this thought randomly popped into my head: In the book series called The Mysterious Benedict Society, one of the characters, Mr. Benedict, is said to tap his nose when somebody answers a question he's asked correctly. This is 100% something James Potter would do.

Just think about it:

"So you're telling me that dementors suck people's fucking souls?"
"Yep!" *taps nose*

"Catching the Snitch earns an extra 150 points, right?"
*taps nose* "That's right."

"Oi! You're a fucking git, you know, Prongs?"
"Affirmative." *taps nose*