I usually play some casual games with the decks i'll use on ranked mode, if i have any doubts i also visit some streamers i like and see how they're doing with their decks so i can get a good grip at the current meta.
I also practise spelling practice a few times in front of a mirror before i go in, definitely a winning scenario !
I often try the deck first on casual mode or with a friend ( if possible ). Very rarely I practice against AI.
However, if you don't care about rank, shoot it in ranked games; more often than not you will match against meta decks or people trying to reach pro rank so it's a good for deck testing.
I also like to create weird (often weak) decks to play in casual, to see what I can do with them, and what opponents do when pushed out of their deck’s routine plan.
Surprisingly, I have probably learned more from a “mindless” monsters pointslam deck than anything else. The beauty of pointslam is that it places a premium on passing, pushing, and bleeding — aspects of Gwent that are unique and subtle.