Randy Barnett has a suggestion for the senators at the Sonia Sotomayor hearings:
Instead of asking nominees how they would decide particular cases, ask them to explain what they think the various clauses of the Constitution mean. Does the Second Amendment protect an individual right to arms? What was the original meaning of the Privileges or Immunities Clause of the 14th Amendment? (Hint: It included an individual right to arms.) Does the 14th Amendment "incorporate" the Bill of Rights and, if so, how and why? Does the Ninth Amendment protect judicially enforceable unenumerated rights? Does the Necessary and Proper Clause delegate unlimited discretion to Congress? Where in the text of the Constitution is the so-called Spending Power (by which Congress claims the power to spend tax revenue on anything it wants) and does it have any enforceable limits?
Don't ask how the meaning of these clauses should be applied in particular circumstances. Just ask about the meaning itself and how it should be ascertained. Do nominees think they are bound by the original public meaning of the text? Even those who deny this still typically claim that original meaning is a "factor" or starting point. If so, what other factors do they think a justice should rely on to "interpret" the meaning of the text? Even asking whether "We the People" in the U.S. Constitution originally included blacks and slaves—as abolitionists like Lysander Spooner and Frederick Douglass contended, or not as Chief Justice Roger Taney claimed in Dred Scott v. Sandford—will tell us much about a nominee's approach to constitutional interpretation. Given that this is hardly a case that will come before them, on what grounds could nominees refuse to answer such questions?