You can check either of the following places to see the combined result of your converter slots:

1. Menu > Factory > Tap the "i" in the upper left corner (to the right of your main spaceship) > Here you'll find each of the contributions of your converter slots listed together

2. Menu > Factory > Swipe left or right on the slots > To the upper right of each of the slot sections you'll see the total effects being applied your current spaceship. For example, in the Energy section, a 6,000 in the upper right of the slots represents an additional 6,000 units of energy storage being applied to the original energy storage capacity of your main spaceship