video: It is never safe to look directly at the sun's rays -- even if the sun is partly obscured. When watching a partial eclipse you must wear eclipse glasses at all times if you want to face the sun, or use an alternate indirect method. This also applies during a total eclipse up until the time when the sun is completely and totally blocked.
During the short time when the moon completely obscures the sun - known as the period of totality - it is safe to look directly at the star, but it's crucial that you know when to take off and put back on your glasses.
First and foremost: Check for local information on timing of when the total eclipse will begin and end. NASA's page of eclipse times is a good place to start.
Second: The sun also provides important clues for when totality is about to start and end.
Learn more: https://eclipse2017.nasa.gov
Download: https://svs.gsfc.nasa.gov/12637
Credit: NASA's Goddard Space Flight Center