Proper testing of SDI cables requires 2 parts - a test pattern generator and an oscilloscope. The generators will output a variety of different signals and patterns, and the oscilloscope will be able to show you the results from the signals it receives. Here is a good introduction to how it's done: https://www.optcore.net/how-to-test-the-sdi-signal/.
Good options for these components are not cheap, so just be aware that a complete setup can easily run upwards of $10,000 USD. Tektronics is pretty much the de facto brand for signal measurement and test equipment, but you can also check out brands like Ensemble Designs, Blonder Tongue, Digital Forecast, Phabrix, and Link Electronics.