How do you know if some is genuine or just playing you?

Only with time.
People are different and have different ways of being and expressing. Just because they don't match our perspective of how it should be doesn't make them fake.
So only time can tell you for sure if a person does not immediately stand out as someone who is just flirting for kicks.

If it feels off, it is. You have your answer. I'm sorry...

Online? On SW specifically? Proceed with caution, trust your instincts, but at the end of the day, it’s still a crapshoot.

There are usually always signs to pick up on that most people just end up ignoring or making excuses for. 🤷🏽‍♀️

i usually just assume they're playing me

Time will tell, and then you still must be careful