We ask for a small number of things. Marketing spend data across all channels, agencies and activities. Commercial performance data — revenue, margin, or whatever KPI the board actually cares about. Any existing measurement reports or agency reviews. And time with three or four people — usually the CFO, the marketing lead, and whoever manages the agency relationship.
We are not expecting perfect data. Part of the value is understanding what exists, what does not, and what that gap tells you.