How can I tell if a civilian is a Faceless?

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.