The beit din may ask the candidate whether he or she feels ready, but I think the answer is largely irrelevant. If you say yes, they're not surprised. Of course you want to move on with your life. If you say no, it's either a) The beis din seeing if you realize there are issues that still need to be handled or b) You're actually ready because you realize exactly how much trouble you're getting yourself into by taking on all these mitzvot. All three answers are more about seeing where your mind is, rather than actually considering your opinion. Yay psychology practiced by unlicensed quacks like batei din and me.
Most people feel "ready" long before there is any discussion of the mikvah. But what "ready" feels like, I'm not sure I could say. It's individual to each person, but it can even differ day-to-day with that individual. Personally, I felt ready, but my "readiness" showed itself as little emotional stabs every time someone asked me why I wasn't done yet and every time I had to point out my differences when I didn't really feel different. However, I consider myself lucky that I knew why I was being told to wait. Not everyone has that luxury (though I would never wish on any conversion candidate outside circumstances that delay a conversion).
I think the better question is how do the rabbis know when someone is ready, but I'm afraid I don't have an answer for that one.
However, I can tell you one thing: If you went out and got a cheeseburger the day before your conversion (knowing it was the mikvah date), you were not ready. I think that's question #2: "Did you eat a cheeseburger while you still could??"
Did you feel ready? How did you know?