I read a book many years ago, and I'm trying to find it again. The story started out with aliens contacting the earth and leaving several devices that basically did a psychology exam of the people who used them. If the person passed the test, they were permitted to settle on a vast planet like thing (I don't remember how it was described, exactly). Anyone who failed was left on earth. The main character of the story takes the test (more than once, I think), and fails. However, he's latter contacted by the aliens and told that while he doesn't have the right psychology to be one of the settlers, he does have the right psychology to be one of the people who goes around and examines sentient species for the opportunity to be given the tests. The rest of the story is about his various adventures as one of these scouts.
Any ideas what the book is?