How a Party Member is Really a Doppelganger

1. When she things you're not looking, she rearranges her teeth. You've trained yourself not to look over when you hear the grating sound.
2. Mirrored copy. Handedness switched, hair parted wrong, sword on the wrong hip. He seems to think you haven't noticed anything.
3. Sense of humor utterly broken; never clowns around with anyone, just repeats what they said in a mutter and tries to laugh.
4. Carefully measures faces of other party members when they seem to be asleep.
5. Once over-rotated head about 150 degrees to spot an ambush, saving the party. No one speaks of this.
6. Drawings of her face later become completely blacked out of their own accord.
7. Accidentally went with party to old hometown. Didn't recognize parents, childhood friends, cousins. Pets didn't recognize him. Parents freaked out. 
8. Can pick locks if no one is looking, by placing palm against lock's mouth and extruding flesh into the mechanism. Became obvious the one time she got stuck.
9. Arrow right through lower left side ribs missed heart and lung when the shapeshifter moved them around, internally, before they could be pierced. Managed to jog 3 miles by torchlight and get out with the rest of the party before anyone noticed the wound. A barber in town helped remove the arrow and was pale as a ghost afterwards, drinking heavily. "That wasn't possible. There was hardly even any blood. When I asked her if there was any pain, she just laughed."
10. (Stolen) They weep after killing a monster. When they think no one is looking, their character stares into the distance and they detect as neither evil nor good, but they are terrified of _sanctuary_. 
11. You can't ever picture her face in your mind. Recognition is possible, you can tell her apart from others, but you can't describe her in words, you can't imagine her face. If you draw her, you find yourself coloring in a dark circle.
12. After Rialto died, Urgwic the torchbearer took up his arms and armor and swore revenge at the raiders who killed the fighter. Urgwic doesn't take off Rialto's old helm anymore, we can never see his face. He talks like Rialto used to, his accent drifted, and despite Urgwic's sandy hair, he's grown the kind of curly black beard Rialto always had. I think he got taller since putting on Rialto's old chainmail.
13. You found an old sack in X's backpack when searching for a spare ration one night. The sack was full of dried out, tanned human faces. At the bottom of the back gently rattled a few teeth.
14. Fell down a crevasse in the Underdark and came back the next day, fine, mostly. Eyes the wrong color. You threw a torch down the crack and could see the glint of her armor at the bottom. 
15. Miscast ESP scroll let you see in his head for a few seconds. All dancing flesh and lies and lives halfway inhabited. Loathes and envies humanity by turns. You know what it's like now to feel your face warp into someone else's. How easily this fools everyone. How far one must go to keep such secrets.