I've heard this one before as well. People have accused me of it. I've laughed. And I've gotten glares in response, because they really think it's true.
Writers... Know What You're Thinking.
I'll save you some time here and just say we don't.
We have a hard enough time figuring out what all our characters are thinking about, without trying to determine what's going on in your head.
Remember that characters try to be as real and dynamic as actual people, but that they never quite measure up. That being the case, unless you're completely one-dimensional and utterly boring, we probably have no idea what's going on in that noggin of yours. Oh, and nobody actually has a brain like that, so you have nothing to worry about.
Simply stated, nobody knows what's going on in your head.
Sure, we're more likely to be able to tell when you're lying or when you're thinking about something in particular, but that's because we know how to read expressions, since we have to describe them on such frequent occasion. But you can't say from that, that we really know what's going on in your head.
Honestly, I barely know what's going on in my own head. Why would I want to know yours?
I might try, if you're important to me or if you're the person I'm basing a character off, but I'd never tell you if that were the case, and I promise to never let you know if I get good at reading your mind.
Mostly because it simply can't happen, and I occasionally enjoy teasing you into thinking I know more than I do. It's a writer thing.