How do you determine if a stock is over-valued? What does that mean? If a willing buyer and a willing seller agree to buy/sell a share of stock, who can say if the share is over-valued? What are some...

How do you determine if a stock is over-valued? What does that mean? If a willing buyer and a willing seller agree to buy/sell a share of stock, who can say if the share is over-valued? What are some of the traditional tools to determine if a stock is over-valued or under-valued?