Good question. If you use Vudu for movies the format is prominently displayed, Amazon and Netflix you have to look for the logo.
A workaround is to use an X-Box for your streaming services. You can set it to Atmos and it has a renderer that will stream everything in atmos, whether it is a native mix or not. The other night I watched a Star Trek movie that was available in dolby 5.1 through amazon video. Set the x-box to atmos and watched it in atmos, much better!