As complicated as girls are, the concept of knowing if one is into you is fairly simple. At least, i believe so. I don't know about anyone else, but i feel that if a girl is into you, it'd be obvious -- unless she was really playing hard to get. But maybe even then, you'll know.
I once heard somewhere that it takes about 2 minutes for someone to figure out if they'll give you a chance or not. I'll just add some time to that, to give it a grace period. So, i'll say that it takes a female about two minutes upon you approaching her, if she will give you a chance or not. Then it'll take about a day or two, for her to really finalize her decision. I say that because sometimes, maybe they'll let you in, give you their number or whatever, but then after talking to you for a while they'll figure out if they'll really give you a full on chance or not. Depending on what type of chance that even is.
The two minutes upon you approaching her, is her analyzing your personal appearance and the way that you had approached her. It's really just a hit or miss. So don't be creepy about it. And not all girls are shallow, as most would think.
A lot of the time, if a girl is into you, i feel like she would over think things. Wonder about you a lot, and most likely would be down to give you the time of day. If she is not into you, she wouldn't care so much to hang out with you, and probably would not go out of her way to do so.
If she is actually replying to your texts messages, or even texts you first once in a while, that does not mean that she's into you. She may be bored, or she may actually think you're fun to talk to; however, it does mean that she's into you. On the topic of texting: if she replies to you with dull responses, she may just not be into you (chances are that she is busy), but a lot of the time if she's into you she would actually try to sound interesting. Or even to sound like she genuinely cares about what you're even saying.
Here's where things do get tricky though. A lot of girls are oblivious about when guys are trying to get at them, and they take it as someone who is just being friendly. So, as a response they're friendly back. They sound super into the conversation, but in this sense it's hard to really tell if they're into you. Lol. If she isn't into you, but talks like she is, you may have been friend zoned. So mention a date, and see how she would respond to that. If she agrees to a date, she is either into you or she's a bitch that is grasping the opportunity of a free dinner. If she starts talking to you less after that date, she may just not be into you.
But, hey i'm not an expert. Everyone's really different actually. In the end, i guess it's just really safe to say that girls are just complicated, period. Good luck.