REST API Documentation - How to tell if a property is mandatory or not

Summary:

How to tell if a property is mandatory or not in the REST API?

Content (please ensure you mask any confidential information):

When we try to create an item Classification, there are plenty of properties in the request body to fill up. For example, "gtmItemClassificationGid",
"gtmProdClassTypeGid" and "gtmProdClassCodeGid" etc.

1. A general question is, how can we tell which property is mandatory, besides looking into data dictionary?
2. To be more specific, last time when we integrated item classification via XML, Product Classification GID is NOT required, as External System wouldn't know Code GID. Now in the world of REST, is "gtmProdClassCodeGid" mandatory?