I purchased a 10 pack of glasses for viewing the upcoming eclipse. They show to be ISO 12312-2:2015 certified. How can this be tested and verified?
The American Astronomical Society (AAS) answers this question on their How to Tell If Your Eclipse Glasses or Handheld Solar Viewers Are Safe page.
Basically they say that since anyone can claim ISO 12312-2 compliance even if they haven't been certified, you should make sure you obtained your eclipse glasses from a vendor whose ISO 12312-2 compliance has been verified by the AAS Solar Eclipse Task Force:
We used to say that you should look for evidence that they comply with the ISO 12312-2 international safety standard for filters for direct viewing of the Sun. But now the marketplace is being flooded by counterfeit eclipse glasses that are labeled as if they're ISO-compliant when in fact they are not. So now we suggest that you make sure you get (or got) your eclipse viewers from one of the suppliers listed on our Reputable Vendors of Solar Filters & Viewers page.
They say that checking eclipse glasses to see if they meet the ISO 12312-2 standard would require a spectrophotometer, which most people do not have access to:
Unfortunately, you can't check whether a filter meets the ISO standard yourself — doing so requires a specialized and expensive piece of laboratory equipment called a spectrophotometer that shines intense UV, visible, and IR light through the filter and measures how much gets through at each wavelength.
So they have assembled a task force to put together a list of manufacturers and vendors of eclipse glasses that they have verified to be ISO 12312-2 certified (their list is longer than NASA's list of 5 manufacturers):
The AAS Solar Eclipse Task Force has been working diligently to compile a list of such vendors, now posted on our Reputable Vendors of Solar Filters & Viewers page. We've checked manufacturers' ISO paperwork to make sure it's complete and that it comes from a recognized, accredited testing facility, and we've personally examined manufacturers' products. We've asked manufacturers to identify their authorized resellers, and we've asked dealers to identify the source of the products they're selling. Only when everything checks out do we add a vendor to our listing. If we don't list a supplier, that doesn't mean their products are unsafe — only that we have no knowledge of them or that we haven't convinced ourselves they are safe.
They also give advice on how to tell if your eclipse glasses are not safe to use:
How can you tell if your solar viewer is not safe? The only thing you can see through a safe solar filter from a reputable vendor is the Sun itself. If you can see ordinary household lights through your eclipse glasses or handheld viewer, it’s no good. Safe solar filters produce a view of the Sun that is comfortably bright (like the full Moon), in focus, and surrounded by black sky. If you glance at the Sun through your solar filter and find it uncomfortably bright, out of focus, and surrounded by a murky haze, it’s no good.