Faceless are dangerous to deal with, moreso when they are concealed as civilians. An outcome of a Dark Event is to have Faceless in missions as civilians, and it's known that even "fleeing" civilians can be Faceless. How do I detect these guys before its too late for my squad?
Some tricks I've learned so far:
- Battle Scanners (can be built after ADVENT Trooper Autopsy) deployed in range of civilians will reveal a Faceless if they are one (and will force them to transform)
- Specialist ability 'Scanning Protocol' will allow you to scan an area an reveal enemies and Faceless
- Firing grenades at a civilian works as well; if you fire at one and DO NOT receive a Friendly Fire warning, it's safe to say the civilian is a Faceless unit (real civilians will always trigger a friendly fire warning)
- Sidenote on firing grenades. When the FF warning comes up you have a choice to cancel your shot. If you don't get it, you will fire a grenade. Watch out if you are on Ironman as all actions are final.
- On civilian rescue missions, ADVENT troops will kill civilians every turn. Civilians cannot take more than 3 damage or they will die. However, ADVENT AI will sometimes shoot a civilian that does not die (meaning it is a Faceless unit).
- Faceless units will automatically reveal themselves if no other enemy troops are alive (ADVENT, sneks, robos, mutons, etc.)
If a civilian is on fire, but still alive, it's a Faceless. Civilians have few enough health that they can't survive being on fire. I suspect the same is true of a civilian that's fallen from a height, but I'm not sure about that one.
IME it's generally pretty simple to tell: you don't know the difference, but the aliens do! If you see a civilian surrounded by active, heavily-armed aliens, and they haven't shot him yet despite having had plenty of turns to do so, shoot him yourself because he's one of theirs. (By "active" I mean that you've discovered a group of aliens in the area and they've done their silly "war cry and then run for cover" thing. If they have a "civilian" in their midst, he's almost certainly not a civilian.)
So far, I've never had one of the "low-hanging fruit" (the civilians that start off right by your starting point with no real chance of the aliens reaching them before you do) be a Faceless. Not sure if that's just luck or if it's deliberately set up that way, though.
Another strategy I use is to approach the civilian. If the civilian is a faceless it will always reveal. At least I've never seen one to not reveal upon me approaching.
Now of course this is risky business and if you are not close enough with a soldier to approach the civilian from the first move then you're in for a treat.
That's why I almost always use a ranger that has both Run&Gun and Implacable. I kill the faceless with a good shotgun and retreat :)