The GPU Usage function of Afterburner or most other video card monitoring utilities is how you check for a CPU bottleneck. In fact, it could be called a CPU bottleneck monitor. Anything below 99% GPU Usage while gaming is an indication of a bottleneck somewhere in your system, most likely the CPU. The lower the percentage, the higher the bottleneck.
Just run an intense game with the Afterburner hardware monitor running in the background while playing a game with VSync OFF. Check the GPU Usage afterwards and see if it is running the 90%+ range.
This is a much more clear and direct way of checking for a bottleneck. If your GPU is having to wait for the rest of the system, it won't be working at its full 99% usage rate. It's actually pretty simple.