How do you tell if a girl wants to be casual with you, even if she's in a relationship?

So there's this girl that I know, who I feel wants more than just friendhip from me. But I can't seem to understand if these are just my assumptions or the actual situation.

She is in a relationship with her long-distance boyfriend. But I like her too and if she's willing I am ready to see where it goes despite the rocky road or even if she chooses her boyfriend in the end.

So what are some pointers or behaviours/actions that you would consider to be suggestive that she wants to be more than friends.

How do you tell if a girl wants to be casual with you, even if she's in a relationship?
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