Just because it has a lot of condensation forming on it, dosen't mean that it's going supersonic.
It seems there is an easy way too tell. If the aircraft is straight and level it is likely to be a transonic phenomenon. When the aircraft is manouvreing then the key may be to look at the wing tips - if there are contrails, then it will most probably be normal condensation due to various factors due the pressure/temperature variations over the wing. If not, again it is most likely to be a transonic effect (which again is based on a number of very complex thermodynamic theories!)...
