POST an XML file and the workflow will return whether or not it's valid XML.
Copy this workflow. Then copy the unique HTTP endpoint in your trigger step.
Then send an HTTP POST request to the endpoint with your XML file. The workflow will return a binary "valid" status if the XML parses correctly, or not.
λ ~/ curl -d @valid.xml https://myendpoint.m.pipedream.net
{"valid":true}
λ ~/ curl -d @invalid.xml https://myendpoint.m.pipedream.net
{"valid":false}