Is it legal to listen one's mind by using some sort of device? How can I find the evidence?

Answer 1

Assuming that such technology exists, it would be the equivalent of a search.

There are constitutional requirements (based on the 4th Amendment) that prohibit the government from instituting a search without probable cause and following proper procedures. But the 4th Amendment only applies to government action.

If a private individual conducts a search, that's governed either by relevant criminal law (breaking and entering) or tort law (trespass, battery). For the device in question, assuming it operates over a distance, there is neither physical entry nor physical contact, so neither trespass nor battery would apply.

Thus, absent a criminal statute (or the broad application of one), the person being scanned would have little recourse except possible under the tort of public disclosure of private facts, assuming those elements are met.

Answer 2

Foil hat. And, good news, it is legal. Stupid maybe, but legal.