You can go to the SteamDB page for a game, click App Info on the left, then look for the "Detected Technologies". This will usually tell you what they're using if it's not a custom engine. You can use the Augmented Steam or SteamDB browser extensions to get a direct link to the SteamDB page from a game's store page.
Also, SteamDB has a page here with aggregate data of how much each detected engine is used across Steam. Unity currently accounts for over half of the games using known engines (snapshot).
You can go to the SteamDB page for a game, click App Info on the left, then look for the "Detected Technologies". This will usually tell you what they're using if it's not a custom engine. You can use the Augmented Steam or SteamDB browser extensions to get a direct link to the SteamDB page from a game's store page.
Also, SteamDB has a page here with aggregate data of how much each detected engine is used across Steam. Unity currently accounts for over half of the games using known engines (snapshot).