What needs to be checked will depend on the method of manufacture. The requirements for castings of iron or steel are considerably different from fabricated and/or welded fittings.
Some makers require much more diligence or Quality Assurance by the buyer as Quality Control on the part of the manufacturer can vary from excellent to non-existent.
The country of origin can make a big difference in the degree of QA that might be required.
QA can be provided by a third party as previously mentioned.
It should be noted that QA inspection standards vary from country to country. In many places all that is done by third party QA is to observe and record, with no enforcing of the buyer's requirements. This means that defective items are noted but not rejected and end up being shipped to the buyer anyway.
In view of this, engaging the services of a third party inspection company that is capable and willing to provide the level of QA required is essential. It will probably be necessary to query a few third party companies to find one that will do what you need to have done.
Make sure that the company and especially the inspector(s) are fully qualified to inspect the various stages of fabrication. Ask for company history/qualifications and resumes for the inspectors. We require that inspectors have at least five years experience in the required inspection disciplines, i.e. foundry castings, machining, steel fabrication, welding, protective coatings, etc.
Good Luck,
Yosh