I developed an API callout to a SOAP webservice, the external system return me an error, how can i see XML file generated by a SOAP API callout?
You can hook your request to a mocking service like https://putsreq.com/ or requestbin.
They allow you to view the raw requests.
Make sure you insert that endpoint Remote site settings as well.
Once you do that and do a callout, you can inspect it at putreq website.