In effect, you ask them. However, this is not done in the literalist HN style - that is, you do not say "Hi, may I please hit on you?" or any other such set of words.
Nonetheless, one asks, and some will say yes, and some will say no; some will know they're answering, some won't even notice, and sometimes the asker doesn't even notice that they're asking.
To a number of people, this sounds crazy. As crazy as the idea that some authors cannot be read literally to get the meaning of their words. There are languages encoded within language and this is one such. If one happens not to speak this language, I guess it's dating apps or nothing :/