Required Class: Operative (or any other if you shoot them out of disguise)
Required Ability: Homing Beacon
Ability Unlocked: Rank 2

To get this achievement, the opposing team must have Operatives that are actively disguising themselves. You must then focus your aim on them (holding LT) and reveal them. The game will not do it automatically. If you find another person with your gamertag, then you know they are a fake. If you see someone not shooting the opposing team, or doing any objectives, you can also figure they may be a fake. Is the other team ignoring them completely and shooting you? Suspicious team mates hanging around enemies without being attacked? You come into a zone plagued with enemies and one of your team mates, camping together. That is who you will want to focus your aimer on.

Note: You can also pick any class and just shoot them out of disguise. This achievement may unlock without you even being aware you were doing it