Here’s one way – catch them looking at you.
There’s this guy at work who – when we first met in our training class – kept looking at me. I know this because I was sitting across the room from him but in direct line of sight… and every time I swung around in my chair to face the person who was answering the trainer’s question – I’d catch him looking at me.
At first he’d look away fast, but as the days went by he got bolder and wouldn’t break eye contact. Eventually he started coming and hovering near my cubicle… but uh – yeah… I’m already taken :)