Download and run CUDA z for linux. Its the equivalent of GPU/CPU-z on windows system except that it monitors CUDA performance and other parameters.
http://cuda-z.sourceforge.net/
Blender 2.68 has native CUDA enabled in the system section. Makes a night and day difference when rendering with GPGPU. Render and check the performance with CUDA-z .
CUDA-z will only work with NVidia graphics cards that have CUDA cores.