I agree with PairPlusRoyalFlush, but remember, for all the talk we have about NC, girls understand that stuff too. They invented NC I'm pretty sure. So when you first meet a girl and she suddenly starts texting all the time, you might think that is high interest when it's medium interest. She's gone NC with her #1 plate and is filling time with you. Likewise, later on, when she develops feelings, she will suddenly care enough to start playing games in order to control the relationship. Then she might contact you less but her interest level is actually higher. If you don't remember this, you might have a defeatist attitude and blow it. Always assume they like you until they tell you they will never go out with you.
Good signs:
1) They text a sexy photo of themselves.
2) They use key words like "excited" and "love" a lot.
3) They text a heart, or broken heart when you don't reply.
4) You get a 700 word text message in multiple parts.
5) The number of days per week they are texting you is increasing.
6) They initiate text conversations.
7) They freak out if you don't reply.
8) They text to invite you out for an overnight trip.
9) You can make plans weeks in advance and they show up on time.
10) They show up for a date even though they are sick.
11) They grab your hand and pull you along in the restaurant.
12) They have an orgasm.
13) They remember stories you told about other women and act jealous for months.
14) They say "What are we?"
15) They French kiss you indefinitely.
16) They call after a date, gushing with how much fun they had.
Bad signs:
1) They text "Umm..."
2) They never send long text messages.
3) They go more than 14 days without replying (then it's over).
4) They say "B-bye".
5) They don't even text a thank-you after a date.
(Yes part of it doesn't make sense but women don't make sense. The same stimulus does not always produce the same response. Talk to women who really like a guy and they often they admit that they won't text the guy first, only if he writes to them first. That usually only happens after high interest + sex has happened).