It really isn't a hard conclusion to jump to, when she factors in the dates and the fact this guy is a wizard who can turn into a stag.

Also glasses. That helps with the identifying.

"No frakin' way." She holds up her hands, shaking her head. "You can't be James Potter, nooooo way."

Even if it rings true, even if everything she's experienced since he approached her as a stag, screams that it is indeed James Potter. Dropping her hands, she peers at him.

"....are you?"