He cut out about a square foot of drywall on the ceiling to diagnose the problem.

I am 23 years of age and teach children diagnosed with Autism.

@Jan Farley Was there a leak, and what do you do to diagnose (I am guessing with the tenant) if there may be a leak before sending out a plumber? 

I'm not a doctor, so I have no business trying to diagnose someone, but I think it's seems very likely that mental illness is the root of this issue and that he must be hallucinating.

Send over an HVAC Tech to diagnose, and if this is a prank call, they pay for it, send them the bill.

I have a great realtor who can diagnose a house with the best of them.

He was diagnosed about three months after we closed on the property.

I've never heard of a GC removing the cover of an electrical panel to investigate the circuit branches, which could help diagnose (without paying for a much more expensive electrician) where the problem lies in that "Broken Light Switch".

It's sad, I have ripped out entire bathrooms for being hacked together for both homeowners and investors alike and let's just say any money saved on cheaper labor was lost and then some on having me come in diagnose the situation, demo, and rebuild. 

I even was diagnosed with a learning disability in math, despite that I believe with patience and step by step process that I will understand whatever math formula that's needed for any task, it might take a little while to get it but it will come through.