Signs of how you could tell if my girlfriend is cheating on me with her girlfriend

Same signs as if she was cheating with another guy:
-Being secretive/lying about what she's doing/where she is/who she's with
-Being defensive when you ask her questions about her friend and what they do together
-Unusual behaviour if the three of you are hanging out together

I'm not sure what you would look for, but this question is a great sign that you need more trust, communication and honesty in your relationship. If you don't have trust, why are you together.

Why do you even suspect such a thing?

Please provide more information concerning your suspicions.