If the majority of a users playtime at the time of review was from playing on Deck, the review will have a Deck icon next to it.
The game in the screenshot is SAND LAND. You can view the reviews here to easily see some examples of this new icon.
Unfortunately there doesn’t seem to be a way to filter reviews to show Deck reviews only.
Not all of them. Full scan of hardware is performed only when participating in a survey or requesting a hardware report. An example is Steam on my PC not detecting a SteamVR native headset (goggles icon doesn’t appear) until I’ll launch VR mode.