Is it possible?

Suppose I have the source of a C program that I would like to distribute on the Internet. This program computes some data (perhaps attempting to to do a partial brute force) and sends it's results to a server. The problem is someone might modify the source to simply contact the server saying "ok, my part of the brute force failed" and not even do anything. How can I be sure that the source has not been modified from the original distribution before it is compiled? My original plan was to compute many digits of PI using the new Plouffe formula (allows you to compute any Hex digit of PI in linear time) but perhaps trying to brute force things in this way isn't such a bad idea.