Like what? Details like this are the most beneficial I think.
Pilgrim mentioned some of the more common one's. Having said that I know there is a difference between reading something and experiencing it. It helps to have someone who is a natural point it out in person as you can "know about it" but still miss it. Here are some from experience with examples.
You want to notice what she does when she first notices you. You need to be looking at her: When you first make eye contact with each other does she adjust herself in any way? They usually will fuss with their hair or clothing. Now are there occasions in which women mess with their hair? Sure, no question about it - sometimes it is windy or it simply gets in the way, but it's an awfully big coincidence if they do it when you both lock eyes for the first time.
If they decide to be close to you - this means they want you to say something to them. Consider your surroundings before making the decision. An example: I once went to a bar/lounge with a friend of mine. While we were in line we started talking to these two chicks. Once we got in there after some chit chat I told them I would be right back. I broke off from them and went to the bar to buy a ****tail and sat at one of the tables off to the side for a bit. This establishment had a dancefloor close to the bar area and the majority of people hung at this spot. While I was sitting there drinking this group of girls decided to plant themselves exactly to my left, just a little bit in front. They were no more than two arm lengths' away. This means they were directly in my line of vision and had stood there close to 10 minutes. The majority of them weren't facing me but from time to time I did see some of them would turn their head to look at me briefly.
If they open you for just about any reason. The only exceptions are if you are in a department store or something and a woman at the counter tries to make conversation - in this case she is simply trying to sell you something and you can ignore. Anything else should result in alarms going off. Women don't directly open guys because they don't want to risk the guy bothering them. They prefer that men they aren't interested in stay the hell away from them. An example: I was once at a bar with a friend of mine watching a basketball game. A group of girls came in, about 5 or 6 of them. They had picked some of the stools next to ours. Before sitting down one of them had came over to me and leaned over and said "Heyyyy, what are we drinking here?" I told her what it was and I asked her what the occasion was with her friends. She had told me it was her friends birthday and then after some more conversation I was basically integrated into the their group.
If they outright smile at you - this one needs no explanation
If they have trouble looking at you and look at the floor or something. I posted something like this yesterday. A lot of times interested women will have hard time making eye contact with men they like, because they think the man is far above her in sexual value. I know there is something to this one because when I was a highschooler and sucked with women I would do the same thing. Whenever I saw and had to pass a girl I liked in hallway I would always end up looking down in passing her, every time. I also had this strange habit of scratching the back of my neck when I would notice her. Everyone has certain "quirks" in the presence of people they like/find attractive.
The best way to persuade oneself about all this is to simply take a month or two to just observe. You don't concern yourself about making moves (although do so if the opportunity comes up), just observe. This is in fact what I did. I spent a couple of weeks going to places where I would have to see a lot of women (nightclubs, festivals etc) and I observed what they did. Anything that seemed unusual I noted. After some time it became clear that certain things happened a lot, there were clear behavioral patterns - these things I noted. In the next phase I decided I would open the one's who did the things that occurred more frequently. You don't worry about what you will say, you just tell yourself when the time comes you will say something and it will be the right thing. One of my earliest successes was in a grocery store - I came across a woman who started fussing with her hair when we both first made eye contact. I said something to the effect that it was my first time there (lol) and I was looking for a certain thing and if she could show me where it was. She was happy to do so and did - yes I did get her # at the end.
A lot of these actions are subconscious and women CANNOT help it. It gives them away if you can catch it. Hope it helps and happy hunting.