When editing a web.xml file eclim will default to validating the file when it is written. Any errors will be added to the current window's location list (:help location-list) and their corresponding line number noted via Vim's sign functionality.
Eclim also combines the above validation with xml validation to validate that the file is well formed.
If you do not want your web.xml files validated automatically when saved, you can set the g:EclimWebXmlValidate variable described in the configuration section below.
Whether or not auto validation has been enabled, eclim also exposes the command :Validate to manually execute the validation of the file.