RE: https://docs.mongodb.org/manual/tutorial/upgrade-config-servers-to-replica-set/#prerequisites

The existing config servers must be in sync.

But how does one check?

Traditionally (or internally here in support land), we've used the dbHash command run against each config server directly and then we just sort of "know" which collections to verify. The 'databases', 'collections', 'chunks', 'tags', 'shards' and 'version' collections need to match in the results (all others can be ignored).

Can we document / recommend a method for checking consistency between 3 config servers?

EDIT: fixed the list of collections we usually check for