Request a comprehensive maintenance log and operating hour record. Focus on the structural integrity of the mast and substructure, checking for welds, cracks, or corrosion. It is critical to conduct a Non-Destructive Testing (NDT) report on load-bearing components. For used equipment, evaluate the wear level of the drawworks, mud pumps, and engines. A professional third-party inspection (such as SGS or BV) is highly recommended to certify that the rig meets API (American Petroleum Institute) standards before final payment.