It is very difficult to shorten an action without any telltale signs of it being done.
A good look inside will often reveal differences in finish - either by weld heat, metal differences, and finishing (machine, grind and sanding) marks in the wall as opposed to the rest of the action interior.
Your best bet that it is genuine is whether, as stated above, the numbers jive and the roll stamp on the wall are proper. I cant recall ever seeing a genuine Kurz without the "oberndorf" roll stamp on the left wall.
Also, if it is a complete original rifle, the stock will be numbered to it as will all of the action parts - bolt, safety, magazine etc.