how to validate external file in the server as part of ReadyAPI REST testing?

There's no built in way in ReadyAPI to achieve the desired result of verifying that a file exists on a remote server. If you can adjust the server to have a shared folder and map that as a network drive, you might be able to use a "File Wait" step. That might be less than ideal due to changing how the server works and opening up potential security holes.

You might be able to use groovy, but I am not sure, I've never done that and I am not sure it's even possible.