I just found, that one game in my wishlist and alert list has a demo on Steam.
Currently there is no way to find, that a demo is released for a game, even if it's in the wishlist and alert list. The only workaround is to check each game manually one by one.
My suggestion, add option "Demo" for Alert feature, or global option to send notification for all games in wishlist, or/and in "alert list".
Global option would be better, because if it would be option per game, we'd have to set it manually for all games in wishlist.