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.
People are in general very, very bad at spotting signs of interest. This is not unique to you. - The non-verbal communication channel for "I'd like to get to know you in a romantic fashion" just does not work very well at all.
Trying to become adept at reading it is, of course, possible but unless you have sky high social intelligence to begin with, I do not recommend it.
What you need to do instead is figure out how to express unambiguous, unmistakable interest in a way that does not scare the shit out of potential romantic partners. If someone doesn't do this, the both of you are potentially running around as the unwitting participants in a real-life romantic comedy where everyone is interested, but also assume that the other party is not.
So, someone has to use their words. Be clear, be honest, be flattering (but still honest), don't make it an ultimatum, and leave their lines of retreat open, both literally and socially. Feeling cornered isn't a turnon for anyone.
Rejection is not the end of the world. You are asking a question, that is all, and unless you do so in quite uniquely repulsive a manner, all that will result is that you get an honest answer, which can only leave you better off.