I want to ask a girl I know who’s in a relationship if she is poly: what’s the best way to do so?

Usually the best way to ask people something is to be straightforward, like simply asking her if she’s open to other partners. But I think in this situation, you might risk coming off as disrespectful or invasive. Most women I know would not appreciate having someone they know ask any variation on “So, I know you’re in a relationship, but are you open to other partners?”

And generally, if someone is poly and they want to date someone, they make the openness of their relationship that person’s business anyway. So unless you’re both in a culture where asking if someone is poly would come off as appropriately conversational, or you have other clues that she might be poly, I’d recommend staying out of it.