How do you know? Have people told you this?

Yeah, essentially. I've been living with the same guy for 20 years, and when he reminds me that I've said "A" in the past I can remember having said A, despite believing that I've always believed not-A, and it seems more likely that I'm mis-remembering my own earlier beliefs than that I was lying to him at the time. Similarly, when he reminds me that he's previously reminded me that I've said "A" in the past and I've had trouble believing that, I can remember that conversation, despite believing that I've always believed A.

Of course, it's certainly possible that I'm just being suggestible and editing memories realtime, but it doesn't feel that way. And if I'm that suggestible, which I might very well be, that makes it even more plausible that I've toggled. So I'm pretty confident that he's right.