Find a surface with repeating lines, like the tiles on a floor. Hold the sunglasses a short distance away from your face and cover one eye. Look through one of the lenses at the lines while moving the sunglasses slowly from left to right then up and down. The lines should stay straight as you look at them. If they wiggle or waver in any way, then the lenses are not optical quality and will distort your vision.
Distortion is extremely common in cheap sunglasses.
What you plan to use the sunglasses for determines the darkness of the tint . For outdoor sports such as mountain climbing and snow-skiing, you want a tint that blocks most light. For most purposes, like going to the beach or driving, look for a tint that absorbs or blocks 70 to 90 percent of light. Tints that offer less than 60 percent blockage are mainly good for fashion since they offer only mild protection. Many of the more expensive sunglasses use specific technologies to achieve increased clarity, better protection, higher contrast or to block certain types of light.