If she says hi to you before you say hi to her when you meet her, and you do not know her a lot, she likes you.
If you do not know her at all and she approaches you to chat, even to ask a question like what time is it, or whether you are XYZ, she is opening you.
If she enquires about your girlfriend, or mention anything resembling talking about a possible girlfriend, she likes you.
If you change something on you (haircut, beard, cloth, accessory) and she mentions that she liked it better as it was before, she likes you. It's a subtle compliment.
If you catches her stare and her gaze linger, or she maintains it with a faint smile, a sneer, or any gut feeling you have that she likes what she sees, do the math. Also, if you catch her looking at you and she immediately look elsewhere, she was checking you out. A female doesn't gawk at guys like in movies, in plain sight within meters, unless he is stunningly good looking or high value. It's too obvious, she looks easy, and she surrenders her power to choose. They'll wait that you look elsewhere instead, or when they are too far to be noticed.
It she acts submissive towards you and complies to your requests, she likes you. The more demanding or the more daring things she complies to, the more she is into you.
If she is finding time to spend with you on her own accord, she likes you.
If she constantly leans towards you and laughs even your lamest jokes, she likes you.
If she attempts to find a way to see you alone, she likes you.
If you find her invading your personal space often, or she hovers around you and she seems to be always there when you go somewhere, she likes you. Proximity is a dead-on indicator.
If she keeps touching you, she likes you.
If she follows you anywhere you go when she is alone with you, without even resisting, she likes you.
If she can't stop playing or placing her hair, exposing her neck, scratching her hand, or acting fidgety and nervous towards you, she likes you.
If there is a lot of lighting and her pupils are still heavily dilated, she likes you. Nothing she can do about it, it's reflexive.
If she always look in your eyes with a calm, easy-going face, or lowers her eyes often when you make eye contact, or looks away then looks at you again, whether you are looking at her or not, she likes you.
If suddenly she acts pissy at you for no particular reason at all or for imaginary reasons, or she hates you for no reason at all, she likes you but can't handle it. Girls that don't like a guy don't waste time being emotional on them, they are apathetic instead, they ignore him and avoid him.
If she is friendly with you in public, but you learn she badmouths you heavily to her friends in private, behind your back, it's possible that the girls likes you and wants to dissuade her friends to go after you. That's the kind of toilet room power play that queenbees play in their cliques. Girls are highly competitive and they'll never hesitate pick up fights or say made-up shit about a guy to weed out her competitors, even within their circle of friends. Same if she treats you like shit in front of her friends, but act totally differently when she is alone. In these case, if you like her, approach her when she is alone.
If she invites you in her home, in her own bedroom or she follows you inside your bedroom, with the two of you alone, it's strongly implied that she accepts the possibility of making out or having sex, if you take the lead and don't rush things.
As a proofing method, I asked my girlfriend for each item above whether it was true or not, and she agreed with all.