One of the first things you need to know about witness samples, whose name is quite representative for their purposes, is that you can also find them as coating coupons. Those coupons are not necessarily freebies or purchase discounts, but rather an opportunity to take a peek at their offerings and see for yourself how the product looks, feels and acts, before you actually spend money on it.
Aside from the two names that witness samples carry, there are also two different types of samples. One is called laser grade coupon, and it defines the highest standards of evaluation and quality, while the other one is the typical coupon. The main difference between them was already suggested: with laser grades, you should get a perfect match or replica of the product.
No chips are tolerated, not even outside of the clear aperture or inside the diameter of the optical device! The other one can present such faults, provided they do not affect in any way the performance of the product.
Also important, the seller should provide you wide range of witness samples, for optical windows, coating layers and many more, targeting different specters of radiation: IR, NIR, UV - VIS, and so on. Whatever you want to preview, you should have the opportunity to receive it.
In case that the manufacturer you get in touch with does not have a sample available, he should manifest the availability to create it and deliver it to you in useful time. And this is just one of the many signs of professionalism you should be looking into. Because the surface of any glass or plastic environment that you order has to be flawless, it would not hurt for you to also ask about optical flats.
These instruments are tiny samples of well-polished glass that test how even is an optical surface. You may purchase yourself one and then use it to test the samples you receive from different providers. On the other hand, you may simply ask the person of contact and discuss with him your options for verifying products.
As a rule, pretty much that there is to find on a profile website should be accompanied, aside from the product details, by an available witness sample. The more such samples they display, the more you should look into their offerings and consider starting collaboration. After all, we are talking about transparency, a gold feature in the fields of optics, if you get the metaphor.