In debug mode it displays last_sync date ("ended_at" value) for completed syncs:
Checking for incomplete (stopped) yum sync tasks...
DEBUG: Red Hat Enterprise Linux 7 Server RPMs x86_64 7Server - last sync: 2017-09-21 07:01:51 UTC
DEBUG: Red Hat Satellite Tools 6.2 for RHEL 7 Server RPMs x86_64 - last sync: 2017-12-21 08:57:44 UTC
DEBUG: Red Hat Enterprise Linux 7 Server Kickstart x86_64 7.4 - last sync: 2017-09-21 07:19:35 UTC
No incomplete syncs detected
In debug mode it displays last_sync date ("ended_at" value) for completed syncs: