How does insurance find out how much mileage were on a car if the gauge cluster was stolen and there was no emission done on the car?

They can track it with the Motor vehicles department, or at least use the last reading from that vehicle odometer if you were not the only owner. If you were the only owner, then it becomes a game of Insured VS Adjuster (to be honest) Don't EVER falsify this info as it can come back to bite you BIG TIME! A simple receipt for an oil change will have odometer readings...