How would a criminal defense attorney dispute an officers testimony that a K-9 got a hit and gave P Cause?

They do so by inquiring, in a hearing on a motion to suppress evidence, into the training of both the dog and the handler and his ability to interpret the dog's conduct. I have seen dogs tested in the courtroom to see if they really have the supposed ability and I have seen handlers tested on their ability to determine what has caused the dog to react.