Eyes dilate when they look at you, they seem happier around you than when not, they introduce conversational topics that are about you or them but not other people, open body language (varies slightly by culture) directed towards you in public situations, very open body language in private situations, they laugh at your stupid jokes, they share secrets about themselves, they are enthusiastic about talking about sex, they are prone to affectionate touching (shoulders, chin, and upper arms in particular, at least in the US).
It's actually very easy to tell when most people have a crush on you, you just have to be paying more attention to them than how you are coming off.
Incidentally, having a crush on somebody (and them knowing about it - they probably will) doesn't lower their odds of reciprocating, turning that crush into something awkward and needy and negative for them lowers their odds of reciprocating. And yes, abusing yourself by catering to their every whim is awkward and needy. As before, the major key is to be paying more attention to them than how you are coming off. (That's also the key to sex; don't try to be "good" at sex, because then you're paying attention to what you're doing rather than how your partner is responding. Indeed, it's the key to social interactions in the general case.)
It sounds like you're describing how to recognize a successful relationship. At the "OMG I don't know if s/he likes me" stage, body language is more likely to be tense and stilted, there will be blushing, and stammering, and painful silences, and all the general awkwardness...
By the time of "very open body language in private situations" all the important questions have already been answered.
If you have a crush on someone you usually want to find out if they have one on you too. In my opinion outright asking them is often not a good solution, because if they don't have a crush on you yet it decreases the chance of this ever happening if they know you have one. This believe is based on what I read about love psychology. Hovever I don't really want to discuss the option of outright asking them in this thread, therefore I have not elaborated further how I got to this believe.
The alternative to asking them is trying to interpret signals that they might give you. However to know how many signals you need before you should believe that they are in love with you, you would need the prior. I have not been able to find anything about the prior of someone being in love with you. Therefore my Idea is to do a survey in order to find out how likely it is that a person you know has a crush on you. The plan is to ask the person taking the survey how many people they know well enough to possibly have a crush on them and how many people they actually have a crush on.
I have created a Survey for this and would be really happy if you would participate.
The next stepp would be to discuss how certain signals a person can give you raise the probability of them having a crush on you. That part is quite difficult. I think probably the best way would be to check how your friends react to certain situations and what body language they show you and then, if you find out someone has a crush on you, to look up what he did differently from people who are merely your friends. I am currently not in a good position to do this experiment but if someone wants to try or has results about this to share please do so. However I think this part is less important than finding the prior, because most people have at least a general idea about what certain signals mean from personal experience while at least I have no idea at all what the prior might be.