Much of the diagnosis of flat feet involve the podiatrist examining your foot when you are sitting down and when you are standing on them. They can make a mold of the foot showing the flatness of the arch of the foot. The podiatrist will also look at your shoes to see how they have worn down over time as it is different with flat feet when compared to normal feet.
In unusual circumstances, the podiatrist may use x-rays and the like to diagnose your problem. A plain x-ray of the foot may show arthritis of the foot as a cause of your flat feet. CT scanning can look at the foot in three dimensions and can see exactly where the bones are positioned. If the podiatrist believes you have an injured ligament or tendon, an ultrasound may be ordered of the foot to show what the soft tissues look like. MRI scanning will also show any abnormality of the bones and soft tissue of the foot but is rarely necessary.