Some PNer/s only have it in one place, like the feet and only stays there, others have it in the hands and feet, some progress start off in the feet and goes body wide this is why its extremely important to find a neuro that is a PN specialist, not just a normal neuro.
A skin biopsy won't tell what is causing your pain, it only good for comparison reasons to check if your nerve damage is getting worse or better, but normally you will know that, just by how you feel anyway.
Thermal testing is normally used first, it only a computor that they put a couple of electrodes on your painfull area and see how the nerves react when a cold or hot sensation is given to you through those electrodes, they normally can tell the small nerve damage that way and a skin biopsy is left as a last resort because its time consuming which means expensive.
good luck
Brian