The Los Alamos National Laboratory has a paper explainin how to measure light speed using the ping command included with most operating systems. It’s very interesting.
An additional explaining: light speed is constant (c=3e8, aprox.), but propagation speed is not constant. It depends on the medium (being exact, it depends on the propagation constant, ß, of the medium) and can be calculated using one of the following equations:
For optical mediums a simpler expression is given using material’s effective refraction index (ñ), but I cannot remember the expression now O:-)