There is a subtle indicator that tells you whether or not you're seeing polarized light, and which direction the light is polarized on. Find out about Haidinger's Brush.
Polarization of light has come in handy over the last few years. Light travels in waves, like the kind you would see traveling along a suspended piece of string if you pulled one end rapidly up and down. Most naturally-emitted light waves are oriented every which way - the crests and valleys can be oriented up and down, side to side, or at any diagonal angle.
Polarized light, on the other hand, has all its waves oriented only one way. Light can be polarized when it reflects off something, refracts through something, or it can be emitted already polarized. One of the easiest ways to polarize light is just to make it go through slits oriented perpendicular to the direction that you'd like light to be polarized. All the light that's oriented in the wrong direction will be blocked as it tries to get through the slit. Many pairs of glasses are covered with a coating with tiny, imperceptible slits which do just that.
Windshield and sunglass makers use polarization a lot, to make lenses that reduce glare. When light reflects off water or snow, or the horizontal surfaces of cars, much of it is polarized horizontally. The reflection of a lot of sun makes the surface look like a giant sheet of light which drowns out all the light around it. By making glass with a coating that polarizes light vertically, most of the horizontal glare is choked off, allowing the person behind the glass to see more clearly.
Movies that use 3D use polarization as well. They film the movie from two different angles, present one angle only in horizontally polarized light, and one in only vertically polarized light. By making the audience wear glasses, one lens of which only allows vertical light and the other only allows horizontal light, the movie presents two different images, which the viewer's brain interprets as one three dimensional image.
One of the advantages of polarization is human's lack of perception of it. Some animals can differentiate between light polarized in different directions, but for humans it's just a handy way of filtering out a certain percentage of the incoming light without changing the perceived image.
OR IS IT?
There is one way for humans to tell if they're not getting all the light they're entitled to, and you can train yourself to look for it. Unfortunately, learning the trick is a little like learning to fly in a Douglas Adams' novel. You can't concentrate on it, or think about it, because the moment your brain realizes it's not ‘reality', you can't do it anymore. It requires practice and just the right amount of focus.
And it requires attentiveness. When polarized light hits the human eye, a figure called Haidinger's Brush appears. Discovered in the 1800s by Wilhelm Haidinger, the ‘brush' resembles a pair of crisscrossed bowties; one yellow, one blue. The blue bow runs parallel to the axis of light's polarization, and the yellow is perpendicular. The bows will rotate with the polarization, allowing you to know if it's shifting.
Some people say that red or blue glass will help you spot Haidinger's Brush, but most agree that your best bet is monochromatic light and polarizing filters, since that will help the colors pop. Polarized lenses from glasses and polarized film will help. Rapidly rotating a piece of paper in sunlight is a cheaper way to practice. Or you could go high tech and tilt your head back and forth while staring at a plain white image on an LCD computer screen. Whatever you do, remember that the image will fade when your brain realizes that it's not seeing anything useful. Looking away and then back, will help. In time, seeing Haidinger's Brush will be easier.