From Gamer Escape's FFXIV Wiki, the free Final Fantasy XIV encyclopedia
You need to match the Typical Adventurer
with his impostor. Interacting with the original one (the one who starts this GATE) will allow you to look more closely at them, allowing you to zoom in on their face, upper body, and lower body. Screenshots are a good idea. Take one of each of the angles, and also take one of the weapon they have equipped. Sometimes the weapon isn't shown in the closer examination, so you'll need to move to their side and zoom in yourself. Move around to the Atypical Adventurers. Interacting with one will give you two choices: One to examine them more closely, and the other to choose them as the impostor. Refer to the screenshots you took very carefully! Sometimes the differences can be *very* subtle.