OK, Jack, so aside from the fact that IE's never age, is there any difference between them and normal elves? And how can they tell, anyway? Do they all get the "spot other immortal" skill?
I think they've all officially got Immunity (Aging) and Immunity (Disease), as far as game stats go.
More importantly, most of them recognize each other -- the "big name" Immortal Elves, at least -- because they've been sniping at each other, flirting with each other, fighting alongside each other, dueling against each other, working together with each other, working against each other, and generally bickering and bantering, for
thousands of years.
And they all know Frosty (now) because of her affiliations, who's mentoring her, and that sort of thing. The Immortal Elves that exist, just like the Dragons that exist, are in a pretty exclusive club together, and while there might
also be an Assensing aspect to it (where they recognize the magical potential someone's got, or can just tell there's something "off" about their aura)...I think they mostly spot each other just by knowing each other so very, very, well.