Want to know how much of your own Steam Library is currently compatible with the Steam Deck? Well, there's now a website dedicated to doing exactly that.
It's called CheckMyDeck (great name) and all you need to do is grab your SteamID, enter it and it will do all the work for you. You can find your SteamID easily using this other website. The main thing to keep in mind though, is that compatibility is an ever-changing thing, with Valve adding to the list almost daily. We mentioned only a few days ago how Verified + Playable together hit 520 games, well as of right now it's already jumped up to 591!
As for CheckMyDeck, it's pretty good actually. Giving you a full list with links to each store page, along with a very tidy looking pie to give an overall score:
Another big thing to remember is that a game being Unknown doesn't mean much - it just hasn't been tested yet, and so it might end up being a flawless experience.