I believe natively supported games still receive the "Steamplay" icon, whereas proton-supported games will show the Windows icon, but will install and run on Linux. (assuming proton is configure)
The official whitelist lives here, https://steamdb.info/app/891390/info/ about halfway down in the "SteamPlay App Mappings" section. It has very stringent requirements and is not often updated. ProtonDB may be a better place to get an idea if a game is "playable," as opposed to the "perfect" that the whitelist is aiming for.
Last time I checked by default only a limited number of verified titles is playable with Proton, so if it appears in your list, it should work well.
You can enable Proton for all your games, although then I think there is no way to distinguish a verified title from the others.
