There are several ways, GPU-Z will show you your GPUs memory load, video engine load, bus interface load, GPU load, etc. But wont tell you what exactly is using what. (if you have multiple GPU tasks going at once for example you wont know which is causing the most load)
If you want to instead look at what programs are using the GPU you can use a program called Process Explorer which can break down GPU use by system processes which can be useful in a few situations, generally troubleshooting system instability or similar.