I remember a movie where in a given moment a guy finds/discover something like a helmet which could detect “not human” people with an on screen message telling “non human” when someone was ahead of him and – i don’t remember if it’s about demons or zombies that looked like normal people unless he used that helmet.
I don’t remember more about the movie except that specific scena and that it was in the 80’s or early 90;s.
I am guessing you are remembering: They Live (1988) – https://www.imdb.com/title/tt0096256/
“Invitation To Hell” (1984 TV movie)?
The full movie is on YouTube, I posted it below. There is a scene with the main character using the helmet starting at 1:18:00 of the video.
https://youtu.be/LNfvLtfjfhY