Basically you don't. That kind of information is held very tight to the vest and I've never seen anyone willing to publish it (in fact, employees often have to have special permission from the Purchasing Manager to even see it).
Thank you very much for your comments. A very good friend send me out Valve Vendor Lists for several companies such as Shell, Exxon, Sinclair, Petrocanada, BP America, CBI & Howe-Baker.
NAPB,
about Shell's Vendor List, take a look at thread408-201794 within this Forum.
The TAT mentioned there is now mandatory to be listed in their TAMAP (= Technically Accepted Manufacturers and Products) DataBase, based on a STAR (= Supplier Technical Assessment Record) System...
In general terms, I would suggest you to focus on a certain number of Valve Manufacturers (pre-selecting them with othr criteria) and directly ask them about the qualifications and references they achieved.
Of course your inquiry should not be limited to the paper (i.e. what's written on the certificates) but should try go into the entrails of what those references really mean...