You could ask how she feels about your guys sex life, if shes content or wishes there was something to change. Based off the vibe of her anwser then give your opinion on the matter. That's how I would personally like to be approached about it at least, and not just out of the blue or a random moment either, take the time to ask in the moment and really listen to her and her reaction to the question not just what you want to hear the anwser to be

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