A word for a similar concept as a shibboleth [duplicate]

I'm trying to think of this word that is similar to a shibboleth but slightly different.

It's generally a question asked to someone to indicate something deeper.

For example, if you wanted to know if someone was telling the truth you could ask them a more general question. E.g. If you were at this location, then you should know what color the door is. It's not only used in interrogations but is used more widely.

Another example, if you wanted to know which political leanings someone had, you could has them a question on a specific issue which you could then infer more about without specifically asking.

There is definitely a specific term for this that is on the tip of my tongue and driving me crazy because I cannot remember it! Anyone know what I'm talking about?

The answer is Litmus Test.

Litmus Test Has Scientific Origins

It was in the 14th century that scientists discovered that litmus, a mixture of colored organic compounds obtained from lichen, turns red in acid solutions and blue in alkaline solutions and, thus, can be used as an acid-base indicator. Six centuries later, people began using litmus test figuratively. It can now refer to any single factor that establishes the true character of something or causes it to be assigned to one category or another. Often it refers to something (such as an opinion about a political or moral issue) that can be used to make a judgment about whether someone or something is acceptable or not.