I start with the question "How would you be able to tell that this problem had been solved?" and repeat it two or twenty times in different words until someone actually tries to answer it.

I handed a client my pen and asked whether it was a solution to their problem

Bleakly funny. Thanks for that. I usually retreat (probably with an angry or pained look on my face) when I notice I'm not really being heard. But sometimes it's better to play and explore.