Maintain eye contact, smile, be super receptive when the guy says something to her, laugh at his jokes no matter how unfunny, always jump in an opportunity to be physically close (this one is big), repeated touching.
If you want to know whether a girl likes you, go talk to her and stand a bit closer than normal to her and see how she reacts. If she doesn't like you, she'll subtly move away from you or even straight up say to you to not be so close (happened to me dozens of times). You could also, while talking to her, playfully wrap one arm over her shoulders or around her waist, if she likes you, she'll play along and stay and/or hold your hand on the other side to "lock" it, if she doesn't, she'll be very uncomfortable and probably move away from it.