When the person tells anda that he/she has a crush on you!
Seriously though, here's one. When you're on the bus atau train, and anda notice that the person looks at anda all the time. Finally, when anda atau he/she leaves the train, and that person still looks at you, there's a good chance that person could have a crush on you. I've noticed that, at least.
At one point during this meeting, I stroked my upper lip (because it was itching), she started doing that too. She also showed me her wrists (not sure how intentional). She also faced me dead on (although this was at the end of the meeting so she might have been moving because she wanted to leave). She also moved her hair a bit so I could see a bit of her neck (this might be her just making herself comfortable), due to long hair. Meanwhile, the saat girl, her feet kept moving, sometimes in my direction, sometimes not (she had done this before, the hari before the meeting and without the other girl present). Later, when the other girl left, the other stayed to do her work, I kept moving around the room, lebih to handle my printing, and she kept moving her body to face me (not sure how directly that was meant). And then she let me use an elastic band (although that might just have been a stroke of luck). She also moved her hair to tunjuk a bit of neck (but this isn't always certain to what it implies).