Agreed as a general principle, but in this case it's a matter of power in vs. power out. If the average power in over a reasonable time the average power out, it's producing power. There's not much room for fooling as far as this is concerned, we can easily measure both. If we can objectively measure a claimed ability it's just a matter of having enough unrelated individuals or teams perform the measurement that a reasonable doubt is defeated. All that's needed here is a search for experimental flaws. If they're truly metering power in and power out, the rest is a matter of what the numbers say.
"Psychics" and those claiming magical powers over subjective matters, that's where magicians shine. When the test involves a judgement of the mind, bring in those who make a living by fooling it.