You can ask many libertarians what makes a libertarian and you will get many different answers. But perhaps the most common answer is that you have to advocate the non-aggression principle (NAP) in order to qualify as a libertarian.
If I have ever said this in the past, I take it back. I don't really like this definition, even though I generally consider myself an advocate of the NAP (not to be confused with sleep). Ironically, I think there are many libertarians who will say they advocate the NAP, but really don't.
If you advocate the NAP, then this means you have to be an anarchist or panarchist. I suppose you could also be a minarchist if you also advocate that tax collections can only be strictly voluntary. Otherwise, you are technically violating the NAP.
If you believe the only function of government should be for police and the courts, then you are still violating the NAP if people are forced to pay for those things.
For this reason, I do not disqualify someone as a libertarian if he doesn't fully hold to the NAP. I am not going to eliminate someone like Ron Paul from being categorized as a libertarian.
Lew Rockwell recently posted a piece written by Murray Rothbard in 1977. Rothbard was a self-described anarchist, but he did not believe you had to be an anarchist to be a libertarian. He just believed you had to be a "radical in the sense of a deep commitment to the spirit of liberty and anti-statism."
I basically agree with Rothbard's reasoning and his definitions. I look for people who are principled and consistently opposed to government solutions. If someone believes in a government court system but opposes every other piece of the state, I am certainly not going to shun this person away as a non libertarian.
Defining a libertarian is not easy. It is highly subjective and everyone has their differing viewpoints. I can't say that I have my own special litmus test for what makes someone a libertarian. I certainly have my litmus tests for someone not being a libertarian.
There are way too many people today who call themselves libertarians, but whom I would not classify as such. You need to be a radical. You need to want drastically reduced government. You need to demand radical solutions that do not involve state action in any way.
I like the Libertarian Party's pledge. Anyone who truly believes in this pledge is probably a libertarian. Unfortunately, many people sign it who don't really believe it.
But I have concluded that you can still violate the NAP and be a libertarian.