Ukemi over time
Recently, a new aikidoka asked me this question "How to I learn to take Ukemi?" My answer, "I can't really tell you, it just kinda happens". That was about the best answer I could give.
Ukemi I've learned is something that involves how well one connects with their partner, and more importantly, themselves. Good ukemi, if I understand correctly, is about being able to adapt to any situation to the the best of our abilities. So to get better at ukemi then, I guess the one thing that has helped everyone I've talked to, is having watched other people take ukemi. And then, only after you try it time after time, it becomes much more apparent. To think of it analogically, its kinda like learning to roll. Sure it doesn't happen overnight, but if you pay attention to a sensei or sempai, it comes to you. Also, it a matter of your limits physically and mentally.
So, as far as I understand, you know you are good at taking ukemi, when you focus more on your connection with you partner than on what position you will end up in or how you can break the hold. Someone who is good at ukemi then, is someone who give their partner all their energy and trust as to execute technique with them. Everything else, like all aikido, will come with time.