Question: Can Satan read your thoughts or just hear your spoken words?
Answer: We must be careful not to attribute to Satan attributes that belong to God. God is omniscient–meaning He knows all things. Satan does not. Satan only knows what he can find out, or what he has already learned. God is omnipresent–He is able to be everywhere at all times. Satan is limited spatially. He can only be one place at a time.
Therefore, you can deduce that Satan cannot read your mind. He can hear what you say, but you cannot assume that he is listening to everything you say. I even question his ability to listen to more than one person at a time. I may be wrong on that, though.