If you lack the time, resources, expertise, or will to do full usability testing, don't give up. You can do one thing that will be a huge first step:
Draw what you're talking about.
That's it. Simple, huh?
You can use a whole lot of words describing something. But if you just draw it out, you'll start to see some of the big problems. It might or might not be the most usable thing in the world, but shy of doing actual usability testing, it's a good start.