Eyes

It's the eyes, you know. Even when they're really good at controlling their body language, their eyes give them away.
You ever watch a cat hunt? Yeah, so their eyes, once they spot the toy or that little mouse, they never look away, their eyes are fixed on that thing.
It's just like that. So it's pretty easy to pick out who's on the hunt. Give it a few beats, cause you don't want to drop a guy who's legit just watching his friend walk in. But if the target doesn't seem to notice the watcher, or have any connection to them? Yeah, you've spotted your monster.

Stalking is when two people go for a long romantic walk together but only one of them knows about it.

— anonymous.