Evaluators are used generally inside the custom action and indicators, if you are planning to use that inside custom action than you can use the permission attribute , using that only users who ave write access on document will be able to see the action..Below is one example of existing action.
<!-- Edit properties pop-up -->
<action id="document-edit-properties" type="javascript" label="actions.document.edit-metadata">
<param name="function">onActionDetails</param>
<permissions>
<permission allow="true">Write</permission>
</permissions>
<evaluator negate="true">evaluator.doclib.metadata.isLocked</evaluator>
<evaluator negate="true">evaluator.doclib.action.checkedOutAspect</evaluator>
</action>