A not very convenient way in the heat of battle is to use https://warframe.fandom.com/wiki/Detect_Vulnerability on Helios. But you could do it once for science and then you know

Also Banshees Sonar skill

Use the Zenith’s alt fire and spawn a demolyst in the simulacrum to see its head. Ignore Juggernaut, it doesn’t have a head even though a spot is maked. You can double check with a weapon that has an on headshot bonus like the Knell.

The Scourge works too, but they ruined it with the speargun update, and you can no longer embed it into an enemy.

As for Archons, Nira’s Snake Head, Boreal and Amar are just their regular heads.

*Forgot, if you’re asking “where is an enemy’s head” because you’re having issues with the Perigale, it is currently very inconsistent client side. https://imgur.com/ok0JVL1

The weird ones are moas and a couple of the infested units. Moa units the “head” is sort of between the legs (the backpack sometimes also counts). For infested chargers it’s the little round nub between the front legs. Nearly everything else is pretty much where you’d think a head would be.

For the archons specifically I’m not actually sure they have a headshot zone, they’ve got all kinds of mechanics and damage reductions/protections. E.G. they’re immune to armor strip. It would not surprise me in the least if they have no headshot zone and are immune to weak point generating abilities like Helios or Sonar.