Control Panel
Just open up the Control Panel > Uninstall Programs and features > Programs and features > Find Microsoft Azure AD Connect. From there you could identify the version.

Locate Azure AD Connect file
You could locate the Azure AD Connect Synchronize file and right click properties to locate the version. (Please refer to reference for further explanation)
Example below;

Help Panel
You could even open up your Azure AD Connect Synchronize services Manager console and click on the help button on the taskbar.

References: