Simple question, right? How can we know when there's something in our software application or website that looks unusable? It's easy:

1. Check your user profiles and find out if there's too many unanswered questions.
2. Figure out whether it's something that you, as a designer/developer, can solve by your own or not. If so, then you got it!

Seriously, if you cannot wait to a thinking-aloud user test, try to "listen" what your personas would ask when they're in front of the desktop.
It works for me when I cannot see, in a first sight, from where the smells is comming.