Run effective and reliable A/B tests.
Ask about the process they used to set up A/B tests.
Ask how they identified the problem to solve through A/B testing.
Ask what kind of supporting data they collected.
Ask how they pulled the insights out of the data they had.
Explore how they came up with all the variations to test against the control - what was the thinking behind each one.
Ask that they explain what went on behind the scenes to get all the variations implemented.