I came across a problem the other day where a process that relied on several external XML documents was failing due to a combination of factors:

1. The document had been hand-edited
2. The document contained UTF-8 characters
3. The document had no encoding directive or BOM to identify it as containing UTF-8 characters

So, I knocked together the following PowerShell module to validate XML documents from the command line:

function Test-Xml {
    param([string[]]$Path)
    
    begin {
        $xml = new-object System.Xml.XmlDocument
        
        function TestXmlDocument([string]$path) {
            $relativePath = Resolve-Path $path -Relative
            Write-Host -NoNewLine "$relativePath : "
            
            trap [System.Xml.XmlException] { 
                $script:valid = $false
                $script:message = $_.Exception.Message
                continue; 
            }
            
            $script:valid = $true
            $resolvedPath = Resolve-Path $path
            $xml.Load($resolvedPath)
            
            if ($script:valid) {
                Write-Host -ForegroundColor Green "OK"
            } else {
                Write-Host -ForegroundColor Red "Failed - $script:message"
            }
        }
    }
    
    process {
        if ($_ -is [IO.FileInfo]) {
            TestXmlDocument $_.FullName
        } elseif ($_ -is [string]) {
            if (test-path -type Leaf $_) {
                TestXmlDocument $_
            }
        }
    }
    
    end {
        if ($Path) {
          foreach ($aPath in $Path) {
            TestXmlDocument $aPath
          }
        }
    }
}

Export-ModuleMember Test-Xml

The function is implemented so that it can be invoked directly or as a filter. It can therefore be used to check a single file:

Test-Xml fileToCheck.xml

a list of files:

Test-Xml file1.xml, file2.xml, file3.xml

or against a set of files as part of a process:

Get-ChildItems -Recurse -Include *.xml,*.config | Test-Xml

The code is probably not as clean as it could be, but it gets the job done.