>>10
What about the 'suppressed smile'? I know it tends to show up more in boys (which might be one of the hints you're searching for by the way) but am not sure if they do count for girls, as they tend to have more "output" on the face expression. This is when they like you, and can't keep from smiling as soon as you speak to them; however as it may seem weird they hide it, but you still get that slightly suppressed smile (slight smirk, smiling eyes).

Also take care about what posture says: is he/she turned towards you? "impressive" stances (shoulders out, back arched so breasts/shoulders look wider, etc), leaning in as "omg this guy/girl is interesting and I want to enter his/her world", etc. These are mostly inconscious but can be conscious and often are good indicators of interest.

By the way, most boys don't take hints. I know them well, and only very rarely notice them when I am interested in the girl (when I am not, however, I do, and know that I am going to defuse that asap).