It isn't actually hard. It's only hard if you are a total arsehole.
If a woman smiles at you, you say something nice but not sexual. Is she still smiling? A little more than before? OK, you can carry on chatting.
Is she wearing headphones, looking at her phone or reading a book? Please fuck off, it is probably a sign she isn't interested. Is she doing a half-smile and one-word answers? Again, kindly fuck off. Are you shouting random shit at her without any indication she is interested? Again, fuck the fuck off. Are you in an isolated place, is her back to a wall, is she alone or in any way 'cornered' by you? Just don't. Are you touching her without permission? Really, really fuck off.
If she says, 'no' or 'I have a boyfriend' or any indication she is telling you to back off, please immediately fuck off. AND this one is important, no woman owes you her time. No, not even if you are 'nice' or 'just trying to be a good guy'. If you have to say it, it's not true. I call this the Posh Spice Rule.