Birthmark? Ummmmm... no.
There are certain physical characteristics but they do not all hold true.
One must look at the way she moves, how she is built, how she holds
herself, her frame and muscle, he body language, her eyes and soul. Also
attitudes and outlooks on a variety of things from personal ethics to
conscience (most elves are definable as "sociopathic" as elves are
commonly not born with a conscience to hold their personal moral system...
they have to develop their own).

*sigh* Okay, IN GENERAL elves are tallish, slim, svelte, wiry of build.
There are exceptions, usually in cases of great strength (external or
internal) or in positional specialization. Cheekbones are generally
pronounced, eyes are deep and expressive, muscle tone is high and strength
is proportionately greater than a human's. They tend to move like a cat,
fox, wolf or other animal and will identify with an animal (much like the
Nor Ind Totems).
Fingers are long and slim, and lips and eyes can easily let you know when
they're up to mischief.

Is that a good start?
Internal/intangible indications are usually better to look for.

Th' elf

--
I'm immune to romantic notions, of course. I'm a block of heavy metal
unshakably planted at the center of a plain of common sense, illuminated
by the sun of reason. Right. Look up. See the swarms of pigs flying south
for the winter?                  "Dread Brass Shadows" - Glen Cook