There’s a few ways.

If you have LiveSync configured, then the SQL agent for the LiveSync clients will display the SQL version at the Instance level:
 

The other is to launch SQL Server Configuration manager and on the services, right click → properties and then go to the Advanced Tab and see the File Version:

You can look up the file version or sql version online to give you the exact release, I normally use https://sqlserverbuilds.blogspot.com/

Alternatively if you have access to the SQL instance just run ‘select @@version’ as a query and it will tell you something like the following:

Then compare it to the list of certified releases in documentation and you’ll know if you need an upgrade. 

https://documentation.commvault.com/2023e/expert/commserve_server_system_requirements_windows_01.html

Hope this helps!

If they have access to SQL Management Studio on the CommServe, they can run the following query:

SELECT @@version

Or via Command Line: 

SQLCMD -S <SERVERNAME>\COMMVAULT
SELECT @@version
GO

Nothing easier than:
 

Then check global repository: https://sqlserverbuilds.blogspot.com/
 

Thank you Jacek!