How to find possible values for xml-configuration-file attributes?

Lets take for example the following tag in the persistence.xml:

<property name="javax.persistence.schema-generation.database.action"
                  value="drop-and-create"/>

I know that the possible attributes can be looked up in the xsd-file(if existing). What I don't know is how to look up the possible values for each attribute. So researching with google is the only option and that takes often long.

This question is meant in general, I know the answer for the example. Any IntelliJ workflows are highly appreciated.