Re: When do you know you are good at ukemi?
Perhaps a little flippant here but....:
Getting good = doesn't hurt during rolling, tobu ukemi etc. Essentially, you can wake up the next morning without bruising or "unusual" stiffness.
Getting really good = as above and can spot your nage's tsuki (openings) during the technique.
Getting really, really good = as above and can take advantage of nage's tsuki with centered, well balanced reversals.....
As has been said, ukemi is more about the quality of your connection (and being able to maintain connection) throughout the technique, rather than how well you can fall over....