...and 'critical thinking skills' (which, without context, means nothing).
I'm not sure what kind of detail you read the article in then, because it describes the students being given an essay-question test. And if you read the links given you'll find out how the test was blindly scored looking for certain specific techniques as evidence of critical thinking: “observing, interpreting, evaluating, associating, problem finding, comparing, and flexible thinking”. They even built in a test for their system, having separate researchers score overlapping samples so that they could make sure they were producing consistent results.
And here's a little bonus:
A large amount of the gain in critical-thinking skills stems from an increase in the number of observations that students made in their essays. Students who went on a tour became more observant, noticing and describing more details in an image. Being observant and paying attention to detail is an important and highly useful skill that students learn when they study and discuss works of art. Additional research is required to determine if the gains in critical thinking when analyzing a work of art would transfer into improved critical thinking about other, non-art-related subjects.
I'm not sure why the summary doesn't include a direct link to the study, as is present in the NYT article, but there you go. There's more detail in there about what they mean by empathy and tolerance (specifically including a measurable decrease in the student's support for government censorship).