They publish a list of year 2000 related issues in their software. Most of the software on their 'untested' list is there because a cursory examination of the source reveals no date manipulation. Also note that the software that shows up 'bad' are mostly very old versions of software with no current problems. Expect year 2000 bugs in GNU software to be minimal. Linux For all Linux distributions, check the GNU year 2000 list against your list of installed packages.
Some general insights follow.
Linux is basically year 2000 compliant, and has undergone as much if not more year 2000 testing than commercial UNIXes. Because the bulk of any Linux distribution is Open Source software harvested from thousands of developers all over the world, it is nearly impossible to get all tests and test results in one place. By virtue of Open Source licenses, nobody can be held legally responsible for bugs. Distribution companies such as RedHat are making a point of mentioning this with regard to year 2000 issues. Look for a small series of upgrades toward the end of the year.