As important as it is to be able to face opponents and figure out what microdecisions to make in a game, testing for consistency alone can be the biggest thing you do. When I playtest by myself, I kind of just pretend to play against a certain deck, where my plays are made by my "opponent" drawing optimally, and playing sub-optimally. Yes, that may not sound like the best idea, but it's an amazing way to test consistency in the form of endurance, ie, the length of the game you can play and get everything you need without dead-drawing often. If you feel really finicky, and want to go the extra mile, make a mark for each game you play, and make another mark in a different section every time you get a dead hand. If your hand continues to be dead, Don't mark it again until it's refreshed, or you "lose" the game. Just remember to keep things realistic, since it's easy to just say "Well, I mean, I can survive 1 more turn with this Pokemon that's had 3 Energy in the active spot for 4 turns".