Every reputable plate shipment comes with a mill test certificate (MTC). EN 10204 defines the types: 2.2 is a statement without test results; 3.1 is a certificate with actual chemistry and mechanical results, validated by the mill's own inspection; 3.2 adds validation by an independent inspector or the buyer's representative.
A 3.1 MTC should show heat number, grade, standard, chemical analysis, tensile and yield strength, elongation, hardness and any special tests (intergranular corrosion, impact, ferrite content for duplex). Match the heat number stamped on the plate to the certificate.
Pressure equipment (PED, ASME) generally requires 3.1 as a minimum and often 3.2. Ask for the MTC before shipment, not after, and check it against your specification line by line.