Do not just say no - show them the impact. "We can add this, but it moves delivery by two weeks and costs X. Shall we proceed?"
Look for: understanding why they keep changing (unclear vision, political pressure, new information), structured change management, regular alignment meetings to reduce surprises, showing the impact of changes on timeline and budget, and building a relationship that allows honest pushback.
Tests diplomatic backbone. PMs who accept all changes are order-takers. PMs who block all changes are gatekeepers. The best PMs facilitate informed decisions by showing trade-offs.