You can check the cpu usage to see if it's at 100 percent while gaming which would indicate a bottleneck, or if you are running windows vista 7 or 8 check the windows experience index scores to spot a bottleneck. I don't think your cpu is the bottleneck, you are running a low end GPU from two generations ago.
The first thing you can do is run an FPS counter (such as FRAPS) while in a game. Run the same level/area on minimum graphics settings, then on max. If the FPS is pretty much the same both times, you've got a CPU bottleneck. Alternatively, you could go into the Windows Resource Monitor and look at the CPU usage in-game. If any core is being fully utilized (not just the entire CPU %) it could be causing a bottleneck. You could do the same thing with a GPU usage monitor (such as MSI Afterburner) and make sure the GPU is not at 100% usage, if you prefer.
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.