You can also measure the end-user behavior without them knowing it. Metrics like "how long did it take users on average to fill in the form?", "how many receive

You can also measure the end-user behavior without them knowing it. Metrics like "how long did it take users on average to fill in the form?", "how many received validation errors on this particular field?" can be used as proxies for how good the design is. Of course, this is only really an option for web applications.