Flat foot Is a flat foot always a bad thing ?
Not at all.
Your foot may be made this way and in children this can be a normal stage of foot development.
It is entirely compatible with normal day to day and sporting capabilities.
A 'good' flat foot is bilateral, painless, has been present a long time, is not progressing, and is flexible(corrects fully). How can I tell if my foot corrects fully? (what does it mean?)
When you stand on tip toe you should develop an arch and your heel move inwards.
From this it is possible to say that the joints, tendons and ligaments (which go to form the arch) are all functioning normally when 'called upon'. Though it often indicates a clean bill of health it doesn't always. There may never the less be the early stages of a problem (for example an early tendonitis of the tibialis posterior tendon). 
A left sided flat foot
The first two pictures show an abnormal response to going onto tip toes(a heel raise), with the heels remaining facing away from the midline of the body.
The third picture shows the normal response with heels moving to face inwards. | 
