By using a SQLite manager to directly inspect the RootsMagic SQLite database.
Totally correct, and I think the following is a non-SQLite way to determine the same thing and that will work in most cases.
For RM's built-in facts, the GEDCOM tag is the GEDCOM standard - BIRT for birth, DEAT for death, etc.
For any other facts, the GEDCOM tag will be EVEN (event) and the "sub-tag" for the EVEN tag will be the name by which the fact is known to RM. If an EVEN tag comes into RM with a new "sub-tag" then RM will create that fact type for you in RM. But the fact type that RM creates for you will not have a default sentence defined. You will have to do that yourself since RM would have no way of knowing what sentence is needed.