Teams that don’t verify the outcome of their work, are heading for failure. Instead, you should understand what you want to achieve and how you can verify if you are working towards this.
Do not rely on assumptions. But also beware of measuring output only, by looking at velocity, sticking to a plan, and assessing predictability. Instead of focusing on what matters, these trigger the teams to do what was told.
Verify your outcomes. It doesn’t have to be all math-like. You can also verify your assumptions by checking with your users and other stakeholders.
Also, have a clear measurable goal and inspect regularly if you are heading towards meeting the goal. This allows you to adjust the course and try other things to achieve the goal.
If you don’t do this, you are no better than a bad waterfall team. As — although often ignored — even with waterfall projects, you should verify your assumptions regularly.