There’s a great article at Co.Design on how to measure usability. It’s an attempt to replicate the Net Promoter Scores that customer service surveys use.
Although asking simple questions for a complex problem such as usability could be viewed as being an extremely blunt tool, it’s also an easy option to quickly survey people with.
And the question that the article suggests using, is:
“How confident do you feel using this system?”
The beauty of this, is that it can be a quick poll, a pop-up box, an inline form – the options are many. And it doesn’t take more than a second or two to answer.