Re: When do you know you are good at ukemi?
You are not asking about Ukemi, you are asking about break-falling.
The latter is only a minor part of Ukemi - recieving a technique correctly.
Which by itself is a only part of being a good Uke.
For the purpose of break-falling, there is one sure reciepe: Train it, and then a little more and again some more. After a few tens of thousands of rolls and breakfalls, you will become good enough to feel safe.
Start training to fall without preliminary planin, just do it slowly.