HDCD, somewhat like Betamax, was a good idea which suffered from a number of unfortunate factors which hindered its uptake. One of these was the way HDCD discs were not always labelled as such and, even when they were, the HDCD encoding wasn’t always done correctly. There were many discs that lit up the HDCD light on players but on which no HDCD features had actually been used! This wiki is far from exhaustive but shows the rather confused state of affairs:
List of HDCD-encoded Compact Discs - Hydrogenaudio Knowledgebase
There was at least one HDCD sampler disc that offered the same tracks with and without HDCD encoding so you could hear the difference - if I see one at a reasonable price, I would be interested to try it:
Mark