WMI is a huge repository. If you want to get to useful information, you will need to specify the name of a valid WMI class. Fortunately, it is easy to search for interesting classes. You can simply use Get-WmiObjectand the -list parameter in conjunction with some keyword. The next line will lists all WMI classes related to video:
Get-WmiObject-list Win32_*video*
You can specify them to check out the kind of information these classes return:
Get-WmiObjectWin32_VideoController