As disabled drivers can park in places forbidden to others just how are the powers that be going to tell the difference?

The fine would not be legal in those circumstances (ie when there's a blue badge holder in the car, any car, and the badge is being displayed). The powers that be would just have to cancel it when they were informed. And they would do so, unless there was evidence the blue badge was being misused (eg when the vehicle wasn't in fact being used to transport the person with the disability.)

Where it's a case of the Congestion Charge in London the disabled exemption is attached to the vehicle nominated by the disabled person - but all it takes is a phone call to switch it to another vehicle for the day.