Chemistry is something that you can't fake. Sparks are visible from the plane, and vibrations travel free, without control. Body language is the universal language of humans. Wrong translation can get you into problems.
If someone makes a quiz, to guess which people are indeed couples, you can guess it by some marks.
They look each other in the eyes, and touch randomly, without a plan.
They smile a lot, because of a mutual impression.
They are arguing about small things.
They did not notice how much time they spent in the same place.
They are cleaning each other. You will move breadcrumbs from his shirt, he will wipe small glitch of chocolate from your lips.
When people try to hide relation or problematic past relation, they will control their reactions. This is an opposite effect because all that you would do in contact with this person is now contrary. For example, you would say hello and shake hands on the street, but now you will turn on your head and walk faster because you don't want to remind yourself of the former relationship.
Angela had the painful relationship with the guy who cheated her. After some time, she found new boyfriend Darren. They visited the local event. There was Pete, her ex-boyfriend. She was in the panic. She wanted to leave, but Darren asked for a reason. Angela wanted to sit farther, and Darren noticed that she is nervous. Pete was looking in her direction, and Angela was looking at him too, with grumpy expression at face. Darren got the point, he asked her about Pete. Men are available to feel when someone is in mind of their girlfriends. They feel hunter's instinct, and you can be silent and cold outside, but they will feel what is wrong. Their pride and masculinity will make a simple equation. It means, their relation is attacked from outside.
If you feel something for someone who doesn't know this, or you wish forgiveness from ex-lover, words are not enough and effective in every case. Actions and body language will show you the way.
Make a smile, move on when she is passing and offer her free chair.
If she is waiting in the line, tell her she can wait in front of you. You have time and she is in rush.
If she is mocked by other girls or guys, be brave and tell her that she was right. Or she is amazing and misunderstood.
Use internet. Make a post with something that she likes. Her favorite song, lyric, movie.
The direct approach is not always desirable. You can lose potential girlfriend if you are too pushy, too persistent or you suffocate her. She needs choice. How she might choose if she can't breathe?