I can't believe no one has mentioned this yet. Get an infrared thermometer ( they usually shine a laser spot on whatever it is that you are trying to measure).
Very good for finding hot spots in racks, also tells thou how hot your coffee is with out having to sip it and burn your mouth.
Actually, you CAN tax illegal things. That's how they got Al Capone: tax evasion.
This is something they've been kicking around for minefield detection for a while. Seed a purported minefield with grass that behaves differently in the presence of explosive, then wait and you can see where to avoid and send the demolition crews.