How can I know if the person I like is gay?

You can do a lot of things that beat around the bush to your question - but the fastest way to know is just to ask! It should be noted that you should make sure its safe to e.g not in a homophobic environment.

Hello! Asking them is the only way to know for sure, unfortunately. If you have a trusted mutual friend, you could ask them. Be careful who you trust, though! Do NOT accidentally start a rumor! If you aren't sure whether they're still in the closet, make sure they'd been safe if they came out of the closet before you ask around. Respect their privacy. If they're online, you can check their profiles and see if they have their sexual orientation anywhere. Hope this helps!

The only way to be 100% certain is to ask, but this can be difficult. If you flirt subtly and they seem to take it well keep going.

The only way to know for sure is to ask them. If you'd like to know if it's "safe" to ask, though, you may first inquire about their views on LGBT in general, to get a clearer idea of how they perceive homosexuality. This way, you'll know if they would be comfortable with you asking them. And maybe, if the person is gay, they may even choose to tell you in that occasion, if you show support to the LGBT realities.