There are multiple ways to verify if UCS is enabled for the users from client side. Following information may help you for reference.
Option-1: Through Lync client configuration
Hold ‘control’ key on the keyboard and right click on the Lync icon available on windows task bar and select configuration information. Drag down and verify contact list provider, it should be UCS as mentioned below.
Option-2: Through Local registry
Open registry editor and navigate to “HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Lync\user's SIP URL\UCS “. Look at the InUCSMode Hkey value and it should be 2165.
Option-3: Through Outlook 2013
Open outlook 2013 and look at my contact list , you should be able to find Lync contacts migrated to outlook.