Before the result
Identify the exact asset, opportunity date, executable price, currency and amount. Decide whether prices are adjusted for splits and distributions. Never mix adjusted prices with separate corporate-action adjustments.
For property, say whether the input is a particular sale, a neighborhood estimate or a metro index. An index move is not an investment return.
At the result
Give the valuation date, data source, formula and exclusions. Round to a precision the inputs deserve. If the current value cannot be refreshed reliably, show the last trusted observation instead of pretending it is live.