The price to earnings (PE) is the first benchmark. You start by dividing the earnings by the number of shares outstanding and compare that to the price per share as a percentage. If the PE is 75 or some such high number, which is to say that if you multiply the earnings per share by 75 you get a number that matches the price, then there are a lot of hopes on a little income, OR the price has tumbled drastically. This is the start of the over/under value determination, but not the whole thing.
Another place to look is debt and equity. A company heavy in debt and short on equity is tremendously risky in these days. This indicates a dearth of intrinsic value, so it might indicate an overvalued condition, maybe. Then there are companies that have a book value (equity divided by the number of shares outstanding) that is higher than the market price.
Finally, for this discussion, you compare the company to its competitors. If you look at Pepsi and then look at Coke, for instance, and see what the difference in earnings per share, equity per share, and other measures of business profitability and potential, then if you determine that for the same dollar of earnings, or the same dollar of equity, etc. one costs more than the other, then you have also made an over/under valuation.