There are a few ways to tell. 1) Is vCenter running on the cluster?  If it is, there's a high probability it's the one deployed by VxRail MGR during the install 2) If you stage the upgrade, do you see vCenter as one of the components that will be upgraded in the list? 3) If you're on 4.7.100 or later, in vCenter, if you go to the cluster, then select "Configure", and selection "System" under VxRail MGR, you will see a "convert vCenter mode" on the right hand side of the screen.  It will have to option to "convert" and it will state "vCenter Mode: embedded".  
How to determine if a vCenter is customer supplied or vxrail integrated?  I inherited a lab with a small VxRail cluster and I want to upgrade vCenter, but obviously would only do that if vCenter is NOT integrated with VxRail.  Thanks.
How to determine if a vCenter is customer supplied or vxrail integrated?