If you log into AtoM with an Administrator account, then you can navigate to Admin (the gears icon in the global header) > Settings.
The first setting at the top of the Settings page will tell you the version. You'll notice there are two numbers, like so:
The first number is the one that will really matter to you - this is major and minor version number.
The second number is the database schema number. Basically, every time that we introduce a change or new feature in AtoM that requires an update to the database (such as adding a new column or table), we also include a database schema migration script that is run during upgrades. Being able to see this number is sometimes useful when things aren't working properly after an upgrade - sometimes people forget to run the database upgrade command-line task as part of the process.
Bonus: there's also a command-line task that can tell you your AtoM version number: