how to tell if a girl is gay?

This might be a long story but please read lol. A girl has just started at my work and she's quite a bit older than me, I'm 17 almost 18 and she's 24. Since she started she was quiet but recently we've got on really well. I'm bi and have a crush on her but I can't tell if she's gay or not. She doesn't seem REALLLY girly but I'm still unsure. Short-ish finger nails, she wears a leather jacket, backpack, timberlands, and has short-medium hair but she does curl her hair everyday. I'm SO bad at reading signs, can you please give me examples of ways you can tell if someone is lesbian?