I've analyzed all files in my library. I can see that those files ripped from HDCD discs are recognized as HDCD. I don't know if there is a way to search the whole library and find those files that are tagged as HDCD by JRiver analysis, but I noticed that on some albums that were not ripped from HDCD, some files (but not all) from the album are tagged "1" as HDCD. This is the case with the super deluxe box set of the first Roxy Music album. All AIFF files were ripped using dBpoweramp, for all three CD discs (I ripped the hi-res DVD audio with DVD Audio Extractor). The first track on Disc One is recognized by JRiver analysis as HDCD. It is the only track on the whole album that is tagged HDCD. Consequently, if I have JRiver playback settings to decode HDCD files, that one track gets played at a much lower volume than all the other tracks on the album.
I don't know if this anomaly occurs with other albums I've ripped from CDs in my library, but I have chosen to disable (uncheck) the "Play as HDCD if possible" option. Obviously this is not the fault of JRiver if some CD albums contain isolated tracks that are HDCD-encoded, but I'd rather not have gross variations in playback volume happening when I'm playing albums in my library, which may or may not have such mastering anomalies.
FWIW, I just did a library view of all my albums that were ripped from discs labeled as HDCD, and examined to see if they were recognized as HDCD by JRiver file analysis. It turns out that NONE of my ripped files from HDCD discs by Bryan Ferry, Joni Mitchell, the Beach Boys, the Doors, and This Mortal Coil are tagged as "1" in the JRiver HDCD metadata field. Only one single album ripped from HDCD is recognized as HDCD (Buffalo Springfield), and one HDCD by Beck on which only one track is coded as HDCD.