Flat feet, do you have them, or do you have feet that flatten?  There are a few ways to tell the difference.  I will discuss a couple of them.  When seated (non weight bearing), put one leg across the other with the outside of your ankle on top of your knee and keep the foot relaxed.  Can you see the curve of your arch on the foot that is crossed over the knee?  If so, then I say that foot is not flat because you can see the curve.  If you cannot not see the curve in your arch then your arch may be low or the foot may be flat. While standing (weight bearing), look at your wet footprint on the floor/pavement.

You should see your toes, the balls of your foot, the heel, and a line that is also wet that connects the heel to the balls of your foot.  If the connecting line between the heel and ball of the foot is as wide as the front of the foot then your foot either flattens or is flat. The lower the arch, the wider the line between the heel and ball of the foot.  The higher the arch, the thinner the line between the heel and ball of foot will be.  Flat or flatten, neither conditions have to be painful. None of these techniques are a definitive diagnostic tool for flat feet.
If you have foot pain and suspect that you may have flat feet or feet that flatten, you should visit a podiatrist or go to a shoe store that specializes in foot function and that performs gait analysis. Either way, flat or flattened feet don’t have to be painful but they can be. There are several things that can be done to care for flat feet or feet that flatten.  Through a store that specializes is proper foot function and gait analysis, getting a proper fitting shoe alone or one that is modified can help.  Over the counter inserts combined with the shoe may also help. Wearing custom arch supports inside the shoes will help to give you foot the support where you specifically need it.  Ask your doctor or podiatrist, pedorthist and or store staff about the benefits of using these devices for flat feet or feet that flatten.