This assumes the following requested meaning:
A question which is asked with the intent of gaining information without asking directly, possibly because the subject would be unwilling to disclose that information truthfully
The following conditions are also assumed:
A single question is asked as part of a regular conversation
The mark is unaware of any test being conducted
The question is only tangentially related to the actual information, if at all
The response reveals the information unambiguously; to cheat such a test requires full understanding and awareness of both the question and expected responses
I suggest the phrase litmus test question, or simply litmus test
While most online dictionaries I consulted do not specify a similarly sounding secondary meaning (it refers more broadly to any kind of test that gives results with one measurement like the actual litmus test which shows the pH of the tested sample), Wikipedia has an article on this technique being applied in politics.
A litmus test is a question asked of a potential candidate for high office, the answer to which would determine whether the nominating official would proceed with the appointment or nomination. The expression is a metaphor based on the litmus test in chemistry, in which one is able to test the general acidity of a substance, but not its exact pH.