I was on a second opinion call for a hole in a HX. Natural draft furnace. I looked inside and saw the hole through rusty but thick metal. Someone took a screwdriver and I guess a hammer and beat a hole in it. The customer said they heard the hammering.
I left it up to them and advised them on how to proceed.
The most difficult part to prove it is not actually seeing them do it, but to assume they did by the noise.
I've also seen the low speed wire cut to the motor so they would have to call them back to check the heat- as well as the white LV wire cut, thermocouples cut and so on.