How to Stress Test a Financial Model?

One of the projects that I've been assigned to in my MM PE internship is to run a stress test on the financial model provided by the investment bank we hired for an exit of a portfolio company. I was wondering if anyone has any experience on this and has any guidance on how to go about essentially questioning and critiquing a model? An example my manager gave was "They projected out that sales reps would be growing by 10% per year, does demand justify this rapid growth?"

I was curious what else I can analyze beyond just essentially questions every basic assumption they make with a why question.