Unlike wireframing, figure out the interface elements of your app without graphic design concerns.
No screen sizes. No layout constraints.
Just the content and its flow.
Get more tangible and accurate feedback than testing mockups.
Mockups are better suited for "how" the interface is styled and animated. That's because they put the test subject into an unnatural state of extra awareness, imagination, and memorization.
Use UI Drafter to devise and test "what" your app should have and do.
Experiment with groupings and progression alternatives to find out the most intuitive and logical flow.
Discover edge cases and blind spots while they're cheap.
Altogether, make decisions with more confidence.
Communicate exactly what you're trying to achieve and make the graphic design phase will feel like a redesign one, familiar and more direct.
Motivate them by showing your intention of building a good app.
Earn their trust with clarity.