tu ask 'em, usually. Its the easiest way. tu can't have both people just kicking their feet around. That relationship won't get anywhere. Its not a bad idea to kick around all the time though. Most crushes are just lust for a few months and subsides.
People usually exhibits different types of signals, depending on the person. They don't vary tooo much, but they're not always the easiest to pick up. --A person like me would talk about my crush very positively. I would compliment the person as much as I can possibly do. But, I also can sharply call the person out if I get frustrated por him/her. The person is probably overly too nice and laughs at your corny culo jokes. <hint, this flirting type doesn't usually work. The other person probably just think of him as a friend. Thats why tu gotta drop the "I amor you" and stuff. --The person is overly shy. He/she looks at tu from far and is very observant with you. This is probably the worst one; if the other person doesn't pay attention, nothing is ever going to happen. The best the person can do is rely on their friends to hint the other person o grow some balls. --The person seems to hate you. Hatred and amor is not too far off from each other. Unfortunately, this also could mean the person legitly hates your guts and not doing confusing culo amor signals. --The person actually asks tu out and seems to pursue you. This person is awesome and straightforward, but if tu don't amor him/her back, this person could potentially become annoying o a stalker. This person usually gets friendzoned and you're a legit ass, but at least tu two aren't getting into a relationship that won't work.
Those are a few I suppose. Relationships takes experience usually. But, tu can be lazy and look it up yourself. Nobody is a mind reader, so don't act like the other person can either.