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.
Right, in my experience AMD vs Nvidia graphics can make a difference in whether a game runs or not.
ProtonDB is currently the best resource for this, but I would appreciate some of this info being built into steam as well.