I used to feel just how you feel sengo. It is hard knowing if ANY girl is into you, white or black. I have made the mistake in the past of thinking that a girl who is friendly and smiles and even slightly flirtatious was INTO me. And I have been wrong many times before when making that assumption. Rejection sucks but as a man you have to deal with it and get used to it and not be afraid of it, otherwise you will never get with a girl.
Sometimes it isn't obvious she likes you, sometimes it is. All the examples people have given you are true and are pretty much common sense. Licking her lips, playing with her hair, crossing her legs toward you, touching you or your clothes for no reason other than she wants to, her hugging you, playing with your hair or your body, catching her staring at you, seeing her looking at your lips or crotch area. All that. Also if a white chick is talking, walking, dressing, and acting hip hop or ghetto then it is clear she is into black guys.
It seems to me you are into the "plain Jane" white girls or you would not be asking this question about how to detect if a white girl is into black fellas. These typical plain white chicks, ones that most would expect aren't into black guys, but you hope they are, are weird to figure out. Some are racist and would NEVER touch a black guy in that way, and some are open to it, but he must be a CERTAIN type of black guy (maybe a bad boy, maybe athletic, maybe sweet and romantic and classy so she can take him home to mom and dad, etc.).
I feel ya man, I love them too. You just gotta approach them and get to know them. Maybe they will make the move, and maybe you will have to. If she rejects you or says she isn't into you, then she just isn't into you (and it could be several reasons, maybe she is racist, maybe she is not into YOU personally, maybe she's a lesbian, whatever!).
If you want to you can use one of my techniques (and I NEVER give my secrets away so feel lucky). You see a chick you like and stand afar but within her field of vision. Look at her until she sees you looking at her. One of three things can happen. One is she can never look back (meaning she's not into you or is involved in a really interesting conversation and is too busy to visually flirt with you). Two is she can look back and give a mean look or even move somewhere else (meaning she is FOR SURE not into you staring at her). Three is she can look back and smile or look back and give you a "hmm what you lookin at, come talk to me" stare (this is always good, it means she is interested). Try it sometime. Just be fearless and go for it man. That's what I do. Jump in that white girl water, it's cold at first but it gets warm fast! LOL