My favorite test is to ask it to invent a magic trick given a set of constraints and props. Because magic is generally published very secretly, surprisingly little of it is in most training sets. Pretty much just the most common method & gimmick exposures people tend to parrot online, but not the theory or exact routines behind those methods.
The worse an LLM is, the more likely it is to suggest literally impossible actions in the method, like “turn the card over twice to show that it now has three sides. Your spectators can examine the three-sided card.” It can’t tell logic from fantasy, or method from effect.