The busiest location is useful evidence
Note whether flies gather around a particular drain, bin, window or food-storage area. Include the time of day and whether larvae or other signs have been seen. Different flies can require different investigations.
Describe maintenance and access constraints
For a business, explain who can arrange access to waste, cleaning or utility areas. For a home, mention any inaccessible spaces near the activity. Ask how the proposed work will address the identified source and what cleaning or repairs may be separate.
Will spraying the visible flies resolve the source?
Visible adults may be only part of the concern. Ask what needs identifying or inspecting before agreeing to a treatment-only proposal.