1. Ask how they would (or did) vote, on this:
"Congress accepts the scientific findings of the Environmental Protection Agency that climate change is occurring, is caused largely by human activities, and poses significant risks for public health and welfare." (link)
(Added Apr 15: Names of US Congressmembers who deny climate change, so their grandchildren can find them)
2. Ask if they see any pattern, to the colors in this image:
(are the colors randomly distributed, or do they change, from one side to the other?)
Refer them to David Brin (then Skeptical Science).
And finally, assess, based on how they've responded to this information.