In some languages, such as C# and NodeJS, we document the best way to verify a connection to MongoDB, via running a command against the admin/default database and sending something like {hello:1}.
This avoids any permission issues you might get for some things like list databases.

While implementing this verification in some sample Ruby code recently (using Mongoid), I wanted to do the same kind of thing as I had done in other languages and found we had no docs on it and had to rely on Google/StackOverflow.

I was able to achieve the same thing so it is possible, it just requires documenting.

Examples in other language docs:
Connection Guide — C#/.NET - https://www.mongodb.com/docs/drivers/csharp/current/fundamentals/connection/connect/
Connection Guide — Node.js - https://www.mongodb.com/docs/drivers/node/current/fundamentals/connection/connect/

We want to make sure that we document how to access the underlying MongoClient.