There are 2 ways to check the version of the installed AAD Connect:

1. Help => About

Start the AAD Connect synchronization console and look for the Menu Item Help, after click on about:

and the About window appears:

2. Installed Software

Click on start and type: appwiz.cpl

after you hit enter the Programs and Features control panel will appear:

as you can see there are 3 entries here:

1. 1. 1. Microsoft Azure AD Connect => this is the AAD Connect Wizard
      2. Microsoft Azure AD Connect Health Agent for Sync => this is the health agent
      3. Microsoft Azure AD Connect synchronization services => this is the sync service itself

sometimes you will see different version for Microsoft Azure AD Connect  and Microsoft Azure AD Connect synchronization services; this means that AAD Connect is upgraded but the upgrade is not yet run to upgrade Microsoft Azure AD Connect synchronization service.

Happy Troubleshooting!