How do you know if theyre interested?

I met two sets of girls yesterday working an event.
One girl was setting up her coffee booth... her coworker, female, told me her name.. but the other didn't...
The other time... co worker and me seem to hit it off, but couldn't tell.. she gave me her number, but i think it was just a job thing...

If they were interested it would be absolutely clear. Right?
How do you know if theyre interested?
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