One good way to find your center is to work with people who have a strong center and have the ability to take your center as well.
When you're uke for these kinds of people, try to feel where they're affecting you. It's kind of a backwards way to find your own center, but it's one way, at least.
While being uke for these people, you may find yourself feeling your breath taken away from you when they affect your center as well. This is just a "side-effect" of their affecting your diaphragm and, I think, is a good indication of when your center has been taken.
The bottom line is, of course, practice...