Keep artefacts a third party can read: signed leases and supplier contracts, bank signature cards, board or member resolutions, approved annual budgets, written performance targets for the manager, and dated instructions or meeting minutes. Reports you merely receive show information flow, not authority.
Build the record while the decisions happen
Oversight evidence is easiest to produce when created in the ordinary course: a monthly management meeting with minutes, a purchase approval above a stated threshold, an annual plan you sign. Store these so they can be retrieved by date. Gaps can generally be supplemented later with declarations and secondary records, but contemporaneous documents carry more weight and cost far less effort than reconstruction.
Create a responsibility table that the parties confirm against the signed agreements. Use reliable business evidence relevant to the actual venture, such as genuine agreements and a reasoned account of the market and operating assumptions. Ask whether the difference reflects a discount, tax, freight, interest, returned equipment, or replacement order, and obtain a dated supplier explanation tied to the final agreement.