I want to second this, because I haven't seen a clear confirmation from anyone I trust (only source that said "only the person who was used for disguise" that I know of is a fairly early build played by one of the IGN guys).
I FOUND IIIIIIIIIIIIIIT!!!! This was taken from a Q&A from Fragworld on May 7th(so it's recent)
Q: I heard that Operatives can spot disguised enemy Operatives. When this happens, Will the auto-pvssyr notify the team, and/or are spotted Operatives "marked" with an icon or indicator, so teammates know who to look out for?
A: Actually, this was a feature we used to have, but we took out. Now, there are very few subtle clues that will let you know if someone’s in disguise. Even if you shoot at them, it won’t be revealed until you actually kill them, so you never know if you’re wasting ammo or actually going after a bad guy, and if the guy in disguise has nerves of steel, he can try to wait out being shot at, to “pretend” that he’s really on your side.
Note, this is in a FF off match. If FF is on, you get XP when you shoot at disguised enemies.
So believe that ho! Lol, but I also remember in the Get Smart videos that the lady said the ONLY way to tell if someone is a disguised Operative is either having the disguisee spot them or by killing them