In the Windows Powershell console, type
[System.IO.Directory]::GetFiles("\\.\\pipe\\")
If your OS version is greater than Windows 7, you can also type
get-childitem \\.\pipe\
This returns a list of objects. If you want the name only:
(get-childitem \\.\pipe\).FullName
(The second example \\.\pipe\
does not work Powershell 7, but the first example does)