"Playtesting" in this case means I get a feel for the deck by running simulated games against a number of imaginary opponents, all of whose actions are dictated by me, and who are physically represented by small plastic toy animal figures: my old alligator Albert, and two recently-acquired tiny Schleich figures about the size of my thumb, one a fox and the other a frog, called (for lack of better names) Foxy and Froggy. They sit at the corners of the table while I pick up their cards and look at them, and play each figure's hand according to what would be in that player's best interest, and try to pretend that none of us can actually see each other's cards. I've been playtesting other games like this as well, such as my evil custom Fluxx deck and Switchboard.
The weird thing is, Froggy's been winning a lot. Way more than Albert, Foxy or me. That tiny little frog is some kind of hustler!