Google Forms are easy to create and can provide you with a snapshot of who's attending your classes, how they're learning about the workshop, and great ideas for ways to improve your class.
Attendee feedback gave us the idea to give more hands-on time during our RefWorks workshops and less instructor-led demonstrations. We've also added an optional 30 minute post-session to our EndNote workshop to troubleshoot specific issues.
If you're interested in seeing the full form just follow the link. You'll want to create your own if you're interested in aggregating data. These questions work well for us, but others might be of more interest for your library.
Are you using another tool to assess your workshop and/or instruction sessions? What works well?