Short Answer
You can't. But you can tell if they're not safe.
Long Answer
There’s no way to tell just by looking at them whether eclipse glasses or handheld solar viewers are genuinely safe, but it’s easy to tell if they're not safe.
Before you use them to look at the Sun, put them on indoors and look around. You shouldn’t be able to see anything through them, except perhaps very bright lights, which should appear very faint. If you can see anything else, such as household furnishings or pictures on the wall, your viewers aren’t dark enough for solar observing.
If your glasses or other viewers pass the indoor test, take them outside on a sunny day, put them on, and look around again. You still shouldn’t see anything through them, except perhaps the Sun’s reflection off a shiny surface or a puddle, which again should appear very faint.
If your viewers pass that test too, glance at the Sun through them for less than a second. You should see a sharp-edged, round disk (the Sun’s visible “face”) that’s comfortably bright, with a dark background all around it. Depending on the type of filter in the viewer, the Sun may appear white, bluish white, yellow, or orange.
If your viewers pass all three tests, they are probably safe. But if you aren’t completely confident of the safety of your eclipse glasses (see below), you should use them only sparingly. During a solar eclipse, look at the Sun through the glasses or handheld viewer for no more than 2 or 3 seconds every 5 minutes or so. This will be enough to observe the Moon covering more and more of the Sun before maximum eclipse, then uncovering more and more of it after maximum eclipse.
Staring at a partial solar eclipse for more than a few seconds at a time, even through perfectly safe solar viewers, isn’t much fun anyway. It’s almost impossible to detect the Moon’s motion across the Sun in real time except with magnification, and you must never look through magnifying optics while wearing eclipse glasses.” Solar filters for camera lenses, binoculars, and telescopes must always be securely mounted over the front of the optics.
What Makes Solar Viewers Safe
Solar viewers are safe if they conform to the ISO 12312-2 international standard for filters for direct observation of the Sun. You can’t just look for a statement to that effect on the product or its package. Anyone can print such a statement, but that doesn’t make it true. The only way to know for sure is to have the product tested at a laboratory that has been approved to conduct such tests by a recognized accrediting body. That's not something consumers should have to do. Instead, it's up to manufacturers to do it.
The AAS Solar Eclipse Task Force maintains a list of solar-viewer/filter manufacturers that we have confirmed have had their products tested by properly accredited labs and shown to be safe. If you buy eclipse glasses or other solar viewers from a company on our list, or from one of their publicly identified authorized resellers, you can be confident that they conform to ISO 12312-2 and are safe to use.
Note that you can't just look for the name of one of the manufacturers on our list on the product or package of your solar viewer. You have to buy directly from a manufacturer on our list or from one of their authorized dealers. Why? Because some unscrupulous companies have printed the name of a manufacturer on our list on their own products to fool you into thinking you've got a legitimate, safe product. Buyer beware!
A genuinely safe solar viewer does more than reduce the Sun's visible light to a comfortable brightness level. It also blocks potentially harmful ultraviolet and infrared radiation, which is invisible. The only way to tell whether your viewer does that is to be certain that it meets the ISO 12312-2 safety standard, and the only way to know that is to be certain that it came from a manufacturer on our suppliers list, as described above.
What if you received eclipse glasses or a handheld solar viewer from a relative, friend, neighbor, or acquaintance? If that person is an amateur or professional astronomer, they're probably OK, because astronomers usually get their solar filters from sources they know and trust (in other words, from the ones on our list). Ditto for professional astronomical organizations (including college and university physics and astronomy departments) and amateur-astronomy clubs.
If you bought or were given eclipse viewers at a science museum or planetarium, or at an astronomy trade show, again you're almost certainly in possession of safe filters. As long as you can trace your filters to a manufacturer on our list or to one of their authorized dealers, you should have nothing to worry about. What you absolutely should not do is search for eclipse glasses on the internet and buy whatever pops up in the ads or search results.
In addition to making sure your eclipse glasses or handheld solar viewers come from a reputable source, make sure they're in good condition:
- If the filters are torn, scratched, or punctured, discard them.
- If the filters are coming loose from their cardboard or plastic frames, discard them.
Indirect Solar Viewing
If you have any concern that your eclipse glasses or other solar viewers might be unsafe, discard them or return them for a refund. If you can’t replace them with genuinely safe viewers, either because there’s not enough time or they’re sold out, you don’t have to skip an upcoming solar eclipse. As noted on the AAS Solar Eclipse Task Force’s eye safety page, there’s an alternative method for safe viewing of the partially eclipsed Sun: indirectly via pinhole projection.
For an example of pinhole projection during a partial solar eclipse, cross the outstretched, slightly open fingers of one hand over the outstretched, slightly open fingers of the other, creating a waffle pattern. With your back to the Sun, look at your hands’ shadow on the ground. The little spaces between your fingers will project a grid of small images on the ground, showing the crescent shape of the partially obscured Sun.
Or just look at the shadow of a leafy tree during the partial eclipse; you'll see the ground dappled with crescent-shaped Suns projected by the tiny spaces between the leaves. A colander makes a terrific pinhole projector, as does a straw hat, a perforated spoon, a Ritz cracker, or anything else with small holes in it.
Keep in mind that you do pinhole projection with the Sun at your back, and what you look at is the shadow of the projector, which will be superimposed with small images of the Sun. Do not look at the Sun through the pinholes! Note too that pinhole projection doesn’t work during totality, but that’s not a problem, because you can look directly at the totally eclipsed Sun –– which is no brighter than a full Moon (and therefore not bright enough to project images through small holes) –– without eye protection, as long as you stop looking or put your solar viewers back on as soon as the Moon begins to uncover the Sun and daylight suddenly returns.