Once you've built your deck, it's time to test it out! Click the deck you want to test and go to the Playtest button on the left side of the screen. This area will let you solitaire your deck, see some opening hands, and show you the probability of drawing a certain card.
To begin a new session, click on the New Game button next to the Playtest header. This will draw you your opening hand, which you can view in the bottom square by scrolling. You can also choose to mulligan, go to your next turn or draw a card.
Each card also has four symbols in the bottom right when you go over the card - view, Return to Top, Return to Bottom, or Graveyard. These allow you to do game actions and fully try out your deck.