"The Love War" (1970 TV movie)?
Here is the plot from Wikipedia:
Two warring planets choose to settle their conflict over which of them
will take over the planet Earth, each sending a trio of soldiers to
Earth to fight to the death. The combatants, disguised as human
beings, can only identify each other by using special visors.
Kyle, one of the combatants, falls in love with Sandy, a woman he
meets during his stay in a small town. In the end, despite cheating by
the other side, Kyle is the sole survivor. But before he can signal
his people he has won, Sandy shoots him with one of the alien weapons.
A dying Kyle then learns that Sandy is also an alien; the other side
has cheated twice. She chose duty to her people over her love for him.
Weeping as she watches him die, she asks him what their half-breed
children would have been. The film’s closing shot shows Sandy through
the visor as she really is: a hideously scarred humanoid. The Earth
faces an orgy of destruction and the extermination of humanity.
You can watch the entire movie on YouTube in 5 different parts, with each part running about 15 minutes long. Below is part 2, and you can see the character played by Angie Dickinson looking through the visor and discovering the man she loves is an alien. This happens at 9:18 of the video.