Currently I try to distinguish between PCL and Text data, does someone know how to distinguish them properly? I have read that there are several PCL standards (PCL1 - PCL6), would this affect the detection?
You could search for manuals of PCL1 to 6 and check the opening and closing tags (they are required). If they are present you know it is pcl. If not, text.
You want more security?
You could write a class(or find one) special for decoding the PCL code and convert it to an image or something. If it fails, it is text.