The speed of light was originally measured by seeing how long it took to get to a mirror and back again. I don't know how it's done now.
Why should the speed of light decay? Why should any constant change? It's sensible enough to assume it doesn't and look for evidence it does. There is no such evidence, despite some very dodgy graph extrapolation that the creationists used to be keen on. I wonder if that one is still doing the rounds amongst the creationists themselves? Tends to happen - discredited arguments still circulate where itching ears are eager to hear them.