Start by setting devMode to true:

<constant name="struts.devMode" value="true"/>

To log missing properties, then:

<constant name="struts.ognl.logMissingProperties" value="true" />

To throw exceptions when a property is missing, or when an exception occurs while evaluating an expression(this will break the pages if there is a problem on the expression):

<constant name="struts.el.throwExceptionOnFailure" value="true" />