Have four-conductor feed at detached garage subpanel, how to verify the ground?
Follow the conduit

I gather the conduit is metal leaving the main panel. And clearly at some point it transitions to PVC.   

It's possible to do that at a coupler, but I rather suspect it's at an intermediate junction box.  I bet the cable from the garage lands in this intermediate junction box, and the ground wire is tied to the metal box at that point. Then, the metal conduit carries it back to the panel.  

You would want to find that box, and put the mark 1 eyeball on that setup and confirm it is so.