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It shouldn't be too difficult to privately ask them. Just a simple, open question. And, if they dont want to answer give them space and time to get to that point of comfort. Dont make it anymore awkward than it might already be for them with theatrics... just keep it simple and understanding.
If I was questioning my partner's sexuality, I would use empathy. Do not criticize and be careful with expressing yourself to him/her. Your partner will remember your reactions to their honesty, so be genuinely humble and understanding.
I'd just ask
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