But you set that choice yourself in the settings. If you want to know what it's shooting then looking at the settings is the simplest and most reliable check. Beyond that I'm not even sure what the DJI GO app preview is for. I never use it. And even if you use it, what are you going to learn from the two different images. The raw image isn't going to tell you anything until it's properly post-processed. The JPG will show you what was captured, although only in low resolution since the full-resolution image is not on the mobile device.
You seem to me to be attempting to contrive a problem where this is none.