I'm sure you've thought of all these, however I offer them to you nonetheless:

Have them keep track of exactly how many gallons they pump into their cars over the next month. Have them start a mini log. Have them date each time they fill up, etc

Have them fill out a qualitative survey (yes/no or something like that):
e.g. Do you compost? (y/n)
Do you ever bike or walk to communte? (y/n)
Do you grow any of your own food? (y/n)
Do you know if there is a local farmers market? (y/n)
Do you purchase your produce at the farmers market?
Do you require gasoline in order to engage in social
activities? (y/N)

Then, say a month after your workshop, you could have people fill out the same survey both for personal (and your reflection) to see if they'd changed any.