To check this, one need to have a Dolby Vision capable TV. If you have such TV, goto Netflix and look for Lost in Space series for example. This series shows Dolby Vision when used with compatible TV and compatible stb.
I've LG 55SM9000 which is Dolby Vision compatible. If I use the TVs inbuilt Netflix app or the Netflix app on Apple TV 4K box, it shows Dolby Vision. If I use Netflix app on my Airtel Xstream stb, it shows it as HDR only. Hope I'm clear.