GPU % usage, not CPU % usage. You can measure GPU % usage with MSI Afterburner, EVGA Precision, etc. If your CPU is not at 99% I don't see how it can bottleneck you in any way. If Skyrim is only using a max of 50% CPU usage on any core, then your CPU is more than enough to handle all tasks Skyrim gives it. Heck, you could run 2 instances of Skyrim at the same time and barely be at 99% usage. Again, check your % GPU usage just to be sure.
It could be that Skyrim is just a terrible console port and refuses to use more than 50% CPU usage. I remember a few older games like that, where it just would not utilize the hardware very good, but I can't believe Skyrim would be like this. Might also be a driver issue with the game of some sort.