It's important to formally track this, even though you can "gut check" this by whether or not they hire you again (the ultimate measure . I like to do the 4 question: "What's going well, what could be improved, any other feedback, would you recommend this service to others?" with examples. This tends to yield a bit more substantive feedback. I may also do a "check-in/debrief" meeting after a particular project to get feedback also.