Well, that depends on the config change you've done. Jetty will output things like the connector's ports, etc. in the stdout output.
Another thing to mention is --dry-run:
See: http://www.eclipse.org/jetty/documentation/current/advanced-jetty-start.html for details.
Then you can enable jmx where you can verify lots of currently applied settings:
http://www.eclipse.org/jetty/documentation/current/jmx-chapter.html
web.xml stuff usually belongs to your application and not to jetty. However using jetty you can override the settings of your web.xml. Consult the documentation if you're interested in this.
So really, you need to be more specific about what setting is not applied to your wishes.
Edit:
You can specify the mime settings in webdefault.xml that is shipped together with the standard distribution of jetty. Here's an excerpt from that file:
<!-- ==================================================================== -->
<!-- Default MIME mappings -->
<!-- The default MIME mappings are provided by the mime.properties -->
<!-- resource in the org.eclipse.jetty.server.jar file. Additional or modified -->
<!-- mappings may be specified here -->
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
<!-- UNCOMMENT TO ACTIVATE
<mime-mapping>
<extension>mysuffix</extension>
<mime-type>mymime/type</mime-type>
</mime-mapping>
-->
Is that the way you've tried it? How do you run jetty? Embedded? Standard Distribution?