how do I use the correct pronouns

Anon
So I've found myself in a pickle. Someone I frequently see in a store, I'm 90% sure was born female. They dress masculine, goes by a name that could be male or female, although MUCH more common for a male, and I'm almost certain I'm noticing a deepening of the voice. I know some lesbians who identify as female but otherwise look male. I don't want to misgender anyone. Should I keep using "they/them" or should I ask which pronouns they go by. I don't want to be rude. 

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