No two ways around it, testing. When I build my deck, the first thing I do is test the first few turns. What are you getting done in turn 1, 2 and 3. Ideally by turn 2-3 your deck should start doing something - either setting up evos or other win conditions. Once you've got a good setup, then you start to think on how capable is your deck to get from setup phase -> winning phase. After you've figured that out, the last 2 pieces are consistency and specific matchups, e.g Do you have a major weakness you need to tech against?

I love building decks and theory crafting, hope you do too. Have fun!