All you need is a suitable programming device that supports your PIC such
as a PICKIT3. Then you can use microchip's integrated programming enviroment
(IPE) which is included when you install MPLAB X IDE to read your PIC device
and save the image to a hex file.
You can then compare hex files to see if there is a diffirence between them.
The configuration settings (fuses) for a PIC contain a flag that prevents
reading a image back from a PIC when enabled, to prevent piracy. So it is
possible that you won't be able to read a PIC when this flag was enabled when it
was programmed.
There is, however, no way to get the original C code back as the hex image in the
PIC only contains compiled machine code, there might be some dis-assemblers tough.