I suspect that the property has tampered with the circuit. How can I find evidence?

Excuse me, is your meter electronic or mechanical? It is normal to have losses. In addition to line loss, there is also electricity meter loss, but the difference of 30% you mentioned is a bit abnormal. I suggest that you send these two watches to the quality inspection department for inspection, but it is best to take pictures of them with a camera or DV machine before disassembling them. It is best to invite a third party to witness it.