But this reminded me another writing tip I've found useful in working out just how well I know my own characters is to interview them. Take questions typically put to people in interviews and let your characters answer them. For example, the "what inspired you to do what you did/become as you are?" can be adapted whether you're interviewing a heroine, a villain (can often be the same character depending on whose perspective you're writing from!) or rebellious fairy godmother (Eileen - and in her case the answer is "I hated magical power abuses and still do so of course I was going to challenge the status quo.").
Interviewing characters this way is a good way of finding out (a) just how well you know them and so are able to write about them and (b) just how strong or otherwise those characters are. A weak character may be appropriate for your story. Equally this may mean you need to develop them further but an interview will show this up. Your characters should be able to "answer" the questions you put to them. And of course your character interviews could make interesting background reading on your website/blog for fans of your stories.
And I've found interviewing characters is a lot of fun. I've found out more than I thought and have found it encouraging to realise my characters do have depth to them. The amount of depth varies but it is appropriate to the character. So it should be with yours.