The American Astronomical Society is warning eclipse viewers that many of the special glasses on the market are not the real thing.
With only one exception, the AAS warns, it is never safe to look directly at the Sun without a special-purpose safe solar filter.
The exception is during totality, when the Moon completely blocks the face of the Sun.
On August 21, this will happen only within the 70-mile-wide path of the Moon's dark inner shadow from Oregon to South Carolina — and even then, only for a minute or two.
Before and after totality, and at all times outside the path of totality, you really need to use a special-purpose safe solar filter when looking directly at the Sun.
How can you tell if the glasses are genuine? The AAS gives detailed information here.