What are their checks and requirements both at the start of the day and after refueling?
That the fuel added makes sense when compared to the gauges on the tank, and is within +/- 4% of the calculated fuel figure. If a discrepancy arises then the tanks are dipped (or magna stick in most cases) for the actual fuel figure.
No drain required as fuel is checked by the suppliers for contaminants prior to and sometimes during refuelling.