There isn't even a difficult way to do it, either in the store or in your library.
One trick (in your library) is to look for games that were updated recently. For example I can see there was a recent update to Grim Fandango Remastered, and it's probably the patch that made it 64-bit. But as Janischan pointed out in another thread, the changelog may not mention 64-bit even though it now is. (To see a game's changelog, click on it in your library, then look in the More menu.)
Games in my library also have a 64-bit tag, but I cannot recall if I created it or GOG. Either way, I know there are 64-bit games in my library that don't have the tag set properly.