The way Nakano reads thoughts is similar to a book, you read the book and you get a picture of it. The picture is different from what the author intended but the key points are followed.
@Ghede That was Mana's hypothesis, but it doesn't actually hold up.
Remember she's seen Toda silently emoting several times. Even with no words, she can get accurate mental images.
Either Toda is a special case, or her power's more complex than Mana thinks. I'm not saying Mana is wrong about her filling in the gaps, just that she clearly doesn't just hear words where Toda is concerned.
The way I see it, she sees it when the person is actively picturing something in their mind; otherwise, the visuals are provided by her imagination. When it's one situation and when it's the other is deliberately left ambiguous by the author.