How do I know if Blender is making use of CUDA to render? I am using Blender 2.69 from the Linux Mint Software Manager on a system with a GTX 950, just checking to see if it will recognize CUDA. After some work, I've gotten the CUDA drivers from Nvidia installed and now there is an option to use CUDA as the compute device in the user preferences. I also selected GPU compute in the properties panel. I rendered a short, one minute clip of video using Cycles Render and monitored my GPU usage in Nvidia X Server settings. It didn't seem like the GPU was being used much (hovering 5-10%) but the video did render a little quicker than I expected.

How can I know for sure if Blender is using my GPU to the fullest to render?
If you have properly set up CUDA and selected GPU Compute in the Render Properties panel then Blender should be using your GPU.  (Though I have seen instances where having two different Blender files open separately and both rendering switches the GPU one to CPU mid-render.)

Probably the easiest way to make sure that Blender is utilizing your GPU is to use some sort of monitoring program.

My Favorite is NZXT CAM, it's pretty sleek and works very well.  CAM monitors and gives graphs of CPU and GPU load, temperature, voltage draws, and fan speeds.  It also shows which programs are using the most load.

HW Monitor is another very popular choice, it doesn't have all the options that CAM has though.  It basically just shows you the current values, minimum, and maximum of each spec.