@lydiaschoch It can change from person to person, it's all in their love language, but usually it's quite a subtly shown emotion in how they pull their words, word choices, how their eyes look at you, or even things they choose to pay attention to regarding you. Usually it's far easier to see once you get to know a person better through passing conversion, but for spotting it on the fly a good rule of thumb can be keeping an eye out for softer looks in the eyes as well as actions or attempts at conversation which try to lead you to pay more attention or to have your mind linger on the person in question. It's not always the case that these will be crushes, but they're a nice tip off which can allow you to further conversation and learn more about the person to be able to tell for certain.

@lydiaschoch It's a sense you gain with time. I used to be quite daft about it, but after spending more time just socializing with people I learned how to tell. Though as of yet there is still no cure for blushing when I sense someone's crush on me.