Probably the same way you tell if any other person does.
Here, do this. Say, "Hi, Emily (or whatever her name is), do you want to go out for coffee?"
And then go and have fun. And if it's going well, say, "I had a really nice time. Can we do this again, but as a date?"
See what she says. Maybe she doesn't like you that way. Maybe she does.
You do not have to have all of the i's dotted and the t's crossed in order to have coffee or a soda with someone.