It's rarely very obvious. I've had women where not only did I think it was a sure thing that she liked me but everyone else did too but they weren't interested when I made a move. I've also had women who came out of nowhere like 'yeah I've been crushing on you for years' and they might as well have hit me in the face with a frying pan because I didn't see it coming. I mean honestly, there's no dead giveaways unless you actually tell them. Smiles, touching, looking, they might make a guy think it's possible but it's not a guarantee.
Usually just talking to me and showing interest. Texts, phone, w/e. Staring with a smile. Physical flirting such as touching me. Going out of her way to help you. Oh, and there is always the more assertive move. Grabbing my ass. But I usually don't like that, so don't try it missy.