In my experience, a lot of times when a sub is accused of topping from the bottom, it reflects more of the Tops insecurity than a particular failure of the sub. That being said, open and honest communication is essential in building a successful relationship. That includes talking about hard and soft limits, experience levels, likes, dislikes, fantasies and nightmares. You both have to talk and both of your opinions should be respected. Just because someone is a sub, does not mean that they no longer have the right to say 'no.'
However, that isn't really what your question is.
My response is this--be polite and speak clearly. Use ‘I’ statements. Keep the focus on the problem you're having, not on accusing or blaming the other person. Example: “I'm not comfortable with this scene. May we take a moment to talk about this?” instead of “You’re out of your mind if you think I'm doing that!”
I hope that helps.