What is in the file, and what is not.

Who to leave out is settled at the same moment as who to include. Names and addresses are folded consistently so comparisons mean something. Nothing in the file is there to reach a total.

Second-opinion pass

Having somebody who did not build the file check it against the brief.

Outlier review

Looking at the extremes of each field, because that is where bad data announces itself.

Field population report

A per-column statement of how much is filled, delivered with the file rather than on request.

If the number you were quoted elsewhere looks too good, send it over.