Summary: Use the Get-Disk cmdlet to identify all bootable devices that are attached.
How can I use Windows PowerShell to see which USB devices are set to be bootable on my workstation?
Use the Get-Disk cmdlet and filter on the BootFromDisk and BusType properties, for example:
Get-Disk | Where { $_.BusType -eq 'USB' -and $_.BootFromDisk -eq $TRUE }