How can I check percent GPU usage on Windows?

Windows includes the Task Manager tool that shows CPU usage in a graphic manner.

This is useful on multiple cases, i.e: to detect rogue sites that use your computer for bitcoins mining. But some of these sites or viruses use the GPU power instead, which is not shown in Task Manager.

What method could I use to detect percent of actual GPU usage?

Accepting both command-line and GUI solutions.

If you use an older Windows version that doesn't show the GPU usage in Taskmanager you can use ProcessExplorer to show the GPU usage:

in system information of in a column (View --> Select Columns --> Process GPU --> GPU Usage):

An opensource solution is Process Hacker which also offers GPU usage in system information (via CTRL+I)

and as column (right click on header and click "Choose columns"):

What method or tool could I use to detect actual GPU percent usage?

You can use Task Manager in Windows 10 Version 1709+

Find out how hard your GPU is working with Windows Task Manager