Hi David,
The speed of light, denoted by
c, is a fundamental property of our universe. It just so happens that photons in a vacuum travel at
c, so we can measure
c by measuring the speed of photons in a vacuum. Photons traveling in a vacuum at speeds other than
c would mean our universe is a much different place than the one we live in, and since our universe is precisely the one we do live in we can be sure there are no photons out there bouncing around at supraluminal velocities.
If it would satisfy your experimental curiosity you could devise an experiment to measure the speed of light of photons arriving from a distant galaxy.
--Percy