This was from the beginning scene of "The Terminator", when Kyle Reese and Sarah first meet.
According to the film's official novelisation, Reese "just knew", based on the fact that the Terminator was significantly above average build, was heading directly toward Sarah and was pushing people out of the way in a fashion that normal humans wouldn't. He was also reaching for what Reese assumed was a gun.
Reese’s mouth had gone as dry as sand. Mechanically, he raised the glass of beer to his lips, allowing himself a sip to moisten them. From the corner of his left eye, he saw a big man move away from the wall in a straight-line path that intersected with Sarah, pushing people out of the way like so much tall grass and reaching with his right hand into the recesses of his jacket. He was the one. Reese knew.
Slowly, he set down the beer and let his hand casually drift to the top button of his overcoat, unsnapping it. His fingers slid over the smooth metal of the Remington 870 and flicked the safety to off.
Interesting, in the original script the scene plays out differently. Sarah takes refuge in a bar (with table service) rather than a nightclub. Reese identifies the Terminator on sight from its silhouette but allows it to sit down in Sarah's booth, and only then takes action when it pulls a gun on her.
ANGLE ON FRONT DOOR as it opens and a figure stands silhouetted briefly against a streetlight.
Reese turns, his eyes flickering to the mirror, the figure.
Close Up Shot - REESE as he mechanically raises his beer. His knuckles are white. He slowly undoes the top button of his overcoat. There is a glint of metal in the shadows within.
Reese turns slowly on his barstool as the figure brushes past him. Out-of-Focus Foreground Sarah looks up.
Extreme Close Up - REESE'S HAND sliding slowly along polished steel, a caress. His finger slips through the triggerguard of the riot gun.
MEDIUM ON SARAH, as the man stops in front of her in close foreground. He sits slowly in the booth opposite her. The angle is OVER HIS SHOULDER.
SARAH (uncertainly): Lieutenant Vukovich?
REVERSE ANGLE - It is not Lt. Vukovich. Terminator sits motionless for a BEAT. Blue eyes so pure and deep. The eyes of a saint, perhaps.
The .45 is out and cocked and AIMED DIRECTLY AT CAMERA, almost in one motion. The bore seems enormous.
BACK ON SARAH, over the gun barrel, her eyes go wide. We hold a BEAT, like a frozen slice of nightmare.
MEDIUM ON REESE as he whips the riot-gun to a hip-firing position, his overcoat falling back with a snap. HE FIRES.