Once upon a time I designed software without using any usability testing. Those were crazy days, full of arguments between team members, which went a lot like this:
Someone: I think it should be like this.
Someone Else: No, I think it should be like this.
Repeat for evvvvvvvvvvvver...
And so, time was wasted.
Then I found myself working at a company that had a usability lab, which I embraced like a bear greeting a picnic basket carrying long lost friend.
Of course, not every company thinks it can afford a usability lab. So it's up to you, yes you, whoever you are dear reader, to step up to the oche (I've never played baseball, so "plate" has no personal meaning for me, however I have played quite a lot of darts). And I've just read a book that tells you how: Rocket Surgery Made Easy by Steve Krug.
Steve Krug is the Thomas Harris of factual books, i.e. it takes him a long time to release a new book. His books are 100% wisdom, and 0% waffle. If you're not familiar with his work, you should be.