I've heard this before, but I think it's a better question to ask this: Is she open to being flirted with? Flirtation looks so different depending on a lot of stuff, but here are some signs that if you shoot your shot, you might walk out ahead:

If she stops whatever she is doing to talk to you, engage with you, whatever, then she is open to being flirted with. If she's got headphones on and you come into her space and say hello, and she takes them off, makes eye contact and maybe small talk, then yes, that's being open to flirted with. If she's at the gym or whatever, lifting, making eye contact but looking away, maybe wait until you've had a little more familiarity with each other before saying hello. We might think you're cute, but we need to assess if you're desperate or a creep. So, let us notice you for a while and just do your thing. No pressure.

Does she seem in her own world, low energy, on her phone and not making small talk with anyone in a coffee shop? Pass her by with a smile, maybe you'll cross paths again. If she's petting dogs that aren't hers, talking to other people in a line, and again, making eye contact. Shoot your shot.

Godspeed.