Check also this and this articles:
"The first noticeable flaw in a replica Panerai is the crown (used to wind the watch). On a replica, it is usually smaller than the genuine and often not put on straight so that it tilts to one side or the other. Also, the crown guard that is distinctive on Panerai watches is often wobbly, and the crown-guard pin that keeps the crown locked will often come loose because it is the wrong size."
"Most Panerai watches use a substance called LumiNova to provide the glow to the dial and hands that allows you to read the time in darkness. Replica Panerai watches normally have a poor quality luminous material applied compared to a genuine watch. The replica won't glow as brightly or as long when charged and viewed in a low light situation."