It's pretty easy to tell, just watch the GPU's load as you game. If the GPU load is maxing out, then the GPU's the bottleneck. If it's not, then it's probably the CPU, but could be a couple other things, like memory capacity and bandwidth.
GPU-Z, Task Manager, MSI Aferburner can all show you your GPU load while you're running the game.
Still, I doubt it, that CPU's plenty fast for games.