How can you tell if a girl wants to dance with you?

The dance situations I'm referring to are like dance floors in a bar or similar. Especially in a crowded situation, I find it very hard to figure out when a girl is dancing near me if she wants to dance with me (or is it another thing like, we just happen to be dancing near each other). All helpful answers are welcome, but I would greatly appreciate it if anyone has nonverbal cues to look for.
How can you tell if a girl wants to dance with you?
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