You need objective, measurable signals if you are concerned that developers might be confused or lying. Otherwise you can just ask us. Most people are happy to tell you what the pain points are in their workflow and how they compare to last year's. Maybe you need metrics to quantify specific complaints like "slow" or "crashes a lot." But I feel like most organizations doing this kind of measurement are looking for some kind of Freakonomics "developers think they want X, but what makes them better is actually Y" when they haven't even bothered to ask about, let alone implement, X.