You have to have spec's in either case and you can't test for the general case. You have to specify for the general case, and the supplier has to provide a product that meets the specifications.

Once you have a set of specifications for what meets the target requirements, you can order a product that meets those spec's. Without the spec's you have nothing against which to test. "Works" is the enemy of "doing it right."

If it appears to function properly when "hooked up" that proves nothing. The proof is in meeting the extremes repeatedly, and observing the effect of exceeding the spec's. If you have a way of generating and exceeding the extremes, then you have the means to test. Before you waste a bunch of time and effort ... and money ... on creating a test, you must first have convincing documentation that indicates that the worst-case conditions will be met. If your suppliers can't/won't provide spec's for the product they sell, they should be in a different line. If you can't produce spec's that characterize what you need, then perhaps you should be.

RE