There are little signs.
The way they look at you, they way they blush when it comes to a personal topic, the way they try to touch you 'unperceivable' and also the way they speak to you.
Sometimes nervous laugh can be a sign as well.
Also, when they are frequently talking about the next time you should meet means they would like to have a more closer relationship with you, rather than just being friends.
I went through this phases a bit more than 7 years ago, so I'm sure in these signs :).