If you work in an office with non-dev's -- like QA's, analysts, regular folks -- ask them. And then listen to what they say.

As a QA, I frequently have the following experience:

me: "This won't make sense to people."

dev: "Oh, but it has to be this way b/c {reasons}"

me: "People are going to get confused. They're going to use it wrong and make errors."

dev: "No, our users are smarter than that."

{6 months later}

dev: "We're redesigning our interface because the users found it too confusing."