For reasons comprehensible only to Washington politicians, it seems that it is unreasonable to question a judicial nominee as to how he or she would rule on some of the hot issues of the day. Seems to me that's like saying when I interview a prospective software developer, it would be unreasonable for me to ask them any computer-related questions. If it's not appropriate to ask a candidate for a position on the Supreme Court about their opinions on issues concerning constitutional law, then just what is appropriate? It's also clear that in Washington, having a "litmus test" is considered bad form, even though everyone has their own well-defined notions of what is and isn't acceptable in a judicial nominee. I guess depending on where you're sitting, the criteria on your side of the aisle are valid qualifications, while the criteria on the other side of the aisle are "litmus tests". Well, just in case the Judiciary Committee would like to know what I think, here is my litmus test.
A Supreme Court Justice should hold as fundamental that our Constitution is one of limited enumerated powers vested in the government to be balanced against fundamental rights vested in individuals (including but not limited to those enumerated in the Bill of Rights). Candidates should be queried on their views of the extent of the Commerce clause, and should be asked to provide specific examples of actual or hypothetical situations that would exceed its reach. (Growing your own marijuana for your own medical use should be one such example.) The ideal candidate will have a healthy suspicion of government's tendency to overreach. This should apply not just to the federal government, but to state government as well. Wherever fundamental rights are concerned, a Justice should not be squeamish about the Supreme Court's duty to protect them against encroachment by state governments as well as federal, with a robust reliance on the Fourteenth Amendment. A lame attitude such as that expressed in Justice Thomas' dissent in Lawrence v. Texas ("If I were a legislator, I would not vote for a sodomy law, but this court has no basis for overriding the state law") is not acceptable.
A Supreme Court Justice should know that the Ninth Amendment was included for a reason, and that it should be understood in the light (not penumbras, mind you, but bright light) of natural inalienable rights to life, liberty, and the pursuit of happiness as intended by its authors. Jefferson and Madison would not have hesitated to think that a right to privacy, to choose how one pursues one's own ideal of the good life, is tantamount to liberty and the pursuit of happiness, and is clearly a fundamental right. A candidate for Justice should not hesitate to think so either, nor to view it as a "fundamental right" in the meaning of Fourteenth Amendment jurisprudence. Anyone who has any second thoughts about Griswold or even Lawrence should get the boot.
A Supreme Court Justice's view of the Establishment and Free Exercise clauses should be well-grounded in the historical understanding of the Constitution's authors that the church and the state must remain separated for the benefit of both the church and the state. Historical misprisions about alleged Christian foundations of our nation, such as those espoused in the benighted neo-antidisestablishmentarian dissent of Justice Scalia in McCreary, should be an outright disqualification for the job. However, a Justice should be sensible enough to realize that there needn't be an absolute firewall between church and state in cases where the government can implement legitimate policy in a way that is neutral among religious beliefs (or lack of them), such as school vouchers, some of which may go to religious schools.
In this post-911 era, a Justice must be extra vigilant of the Bill of Rights, insuring that freedom of speech and of the press is not curtailed, especially in the name of patriotism, and that the rights of the Fourth, Fifth, and Sixth Amendments are not compromised, especially in the name of national security.
A Justice should conscientiously protect the property of individuals, as required by the Due Process clauses and the Takings clause. A qualified candidate should demonstrate an understanding that the Takings clause has some substantive meaning (and the ideal candidate would concur with Justice Thomas' dissent in the recent Kelo travesty).