O.k. make sure that all channels have the same size (if not something is fishy) and that they really cover the same area. Once you are sure that the data sets are 'formally o.k. you should perform a quick quality check. Calibration is real hard core stuff and I recommend to leave that part to the experts, but you should at least check if your data sets are reliable. Some basic checks involve the analysis of a/some corner reflectors (if you have some in the scene) trihedral corner reflectors are widely used as calibration targets. The trihedral structure results in a triple bounce (odd-bounce) scattering behavior. Therefore the cross-polar backscatter should be much much lower than the co-polar backscatter. The amplitudes of the HH and the VV channel should be similar and the phase difference close to zero. If you have no trihedral corner reflectors in your scene you can use a good reflecting surface (make sure that you get enough backscatter). For an overall analysis you can also use a Pauli or Sphere/diplane/Helix decomposition. Just look at the result of the decomposition and ponder if the result is reasonable. Actually I suggest to make the Pauli or Sphere/diplane/Helix decomposition in any case and to try to interpret what you see. You can learn a lot just from fooling around with the data. Once you think you know your data and the data quality is o.k. you can start the real work
In the following I wrote down some simple step by step recipes for the decomposition approaches we discussed before.