I think as Darlyn said if a car is weaving in and out of traffic, and overtaking cars at a speed obviously faster than the rest of the cars, it is safe to assume they are driving above the legal limit and therefore breaking the law. It would not be likely that all other traffic is sitting 10 - 20kph below the posted limit.

Now, it is one thing to make an educated observation that the vehicle is exceeding the legal limit, but saying you can determine by how much over it actually is with the naked eye using 'experience' is at best a guess and should in no way be able to be used as 'evidence' to book/charge the driver for anything other than maybe dangerous/reckless depending on the extent of weaving/tailgating etc....

I personally have no issue with an officer charging you for reckless if you are cutting people off, weaving continuously through traffic at an obvious higher speed (if you drive like a dick you deserve the penalty), but I draw the line at the officer being allowed to give you a speeding fine where he is 'guessing' a speed.

My opinion only...