Litmus is a water-soluble mixture of different dyes extracted from lichens, especially Roccella tinctoria. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity...
Forget tax policy, or abortion, or school funding, or any other distraction from the real argument. The real litmus test for Liberty is support for the Right to Keep and Bear Arms.
The Statists and Collectivists oppose individual Liberty. They routinely propose laws that restrict freedom. They uniformly look upon RKBA with disdain and derision. If you tell me what a person's core position is on RKBA, I can generally tell you their position on any other issue. (And, yes, this includes Harry Reid.)
Try it for yourselves. Pick an issue. Look at that person's stance on the issue (specifically whether it increases or decreases Liberty), and then look at their stand on RKBA issues. In MOST cases if they are anti-liberty, they will be anti-gun as well.