The on board software doesn't show any info about the availability of a Supercharger. Maybe a feature like that will come in the future, but I actually doubt it, as there is no way of knowing how busy a station will be in 10 or 20 min or even further out. There is no way of know how long someone will charge and when they arrive. In most cases Superchargers are too far apart to just re-route. Even if within reach, the extra time it would take to go through a different Supercharger and then back to your original route would be much longer than a few minutes waiting at a busy Supercharger.
Having visited almost 40 different Superchargers and having done tens of thousands of miles on long road trips using Superchargers I can say from my experience, you won't have trouble with a full Supercharger. After 62k miles in my Model S I had to wait at a Supercharger twice. Both locations have been expanded since then and many more stations have been added. Tesla is pretty good about keeping Superchargers available.