(WENY) -- The solar eclipse is just under three weeks away and to watch the astronomical phenomenon on April 8th, you'll need the proper glasses; but how do you know which ones are safe to use?

"On the back of your glasses that you purchase, they should have an ISO number,” said Deborah Dann, Professor and Director of the Eileen Collins Observatory at Corning Community College in an interview earlier this month. She added, “That number kind of ensures that [it] is an official standard for making these eclipse glasses..."

According to the American Astronomical Society, the organization said before the 2017 eclipse, they were telling people to look for the ISO number on the inside part of eclipse glasses. A few weeks before the eclipse however dishonest vendors started selling fake glasses by pulling the ISO logo off the internet and putting it on products and packaging.

The proper eclipse glasses should be pitch black when you put them on and you shouldn't be able to see anything, except the sun, or something else that's bright like the sunlight reflecting off a mirror a shiny metal object.

According to the American Astronomical Society, "You shouldn't be able to see anything through a safe solar filter except the Sun itself or something comparably bright, such as the Sun reflected in a mirror, a sunlight off shiny metal, the hot filament of an incandescent light bulb, a bright halogen light bulb, a bright-white LED bulb (including the flashlight on your smartphone), a bare compact fluorescent (CFL) bulb, or an arc-welding torch. All such sources (except perhaps the welding torch) should appear quite dim through a solar viewer. If you can see shaded lamps or other common household light fixtures (not bare bulbs) of more ordinary brightness through your eclipse glasses or handheld viewer, and you're not sure the product came from a reputable vendor, don't use it."

If the glasses are fake, even if they have the ISO certification, you'll be able to see through them like a pair of sunglasses.

It's also important to note that eclipse glasses have a shelf life of about three years and aren't safe to use after that. So, if you held onto your pair from the 2017 event, don't use them.

Be sure to check your glasses for any damage before you view the eclipse as well.