After migrating to DBStore, how to check it was (entirely) successful

Issue

We are in the process of migrating documents and media from our file-system based store to the database (DBStore)

How to find out if all active documents and media are migrated to the database? Is there a way to do a count?

First of all, if the migration process finishes with success message and it does not log errors, you can be pretty sure that all went fine
When such a migration is done, the data stored in the file system will be from now on stored in the database's dlcontent table (which is empty before the migration). Therefore a good way to verify that the migration has been performed is to verify that after it, there are a lot of rows in the dlcontent table (whereas before, it was empty) - this article might be helpful: How to migrate to DBStore. Regarding performing an exact count, it's difficult to say how many rows should there be exactly. The row count should be somewhat related to the total number of document versions in your system (the dlfileentry contains a row for each document, and the dlfileversion table contains a row for each document version).