I would hire third party inspection company to go and do a shop audit, and then you will get the information you are looking for, plus they have nothing to gain if the shop is not up to your standards... there would not be empty promises from a vendor that wants your business.
Check that what's in the box is what's supposed to be in the box, e.g. with a few dimensional checks.
One thing I would _not_ do is remove protective caps and plugs from threads, unless you have the proper plug and ring gages. Touching pipe threads with calipers is just going to nick them.
Those elements of the manufacturer's approved inspection and test plan (ITP) indicated as requiring the specific attention of the party whom you will represent?
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.
