How can I tell if a civilian is a Faceless?

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?

Solution 1:

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.)

Solution 2:

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.