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?