I'm not sure if she knows that I'm trans. I didn't hint at it intentionally but I think that it's sort of obvious. When I was 11, I told her that I hated having breasts and wearing bras and that I was going to get surgery when I was older. I told her again this year that I hated it and she said that she didn't like it either when she was younger but she doesn't really know the full story. She asks me if I think I want to be a man a lot now but I've always been too nervous to say yes. It's because I've started wearing my dad's clothes and buying my own male clothes. I think she might know that I bind. Then she said that all I need to do is grow a beard and they'll call me Chaz and my transition will be complete... Does she know or am I just overthinking things?
What tone does she use when asking? Is it nervous or joking? Worried or chill? How do you think they would take it? If you think she already knows or that she will take it well, you could tell her sooner rather than later.
You could also tell her you're "thinking about it", even though you're sure, as like an intermediary step.
Even if she's joking though, it would likely be that she noticed some sort of gender variance in you. The wording does come off as more of a joke though. Does she seem like the kind of the accepting? Does she usually see trans people in a positive or a negative light? If if you feel safe coming out to her then I have a feeling she might not be all that surprised. You could also start saying "yes" to her questions, I think that would be a pretty major hint. DO consider your physical safety if you're thinking of coming out though. If you feel at all like you might risk being kicked out or anything, wait.