Historical rate of return on stocks is about 7%. So in theory, a stock trading at a price to earnings ratio of 14 or less is good value, whereas a stock trading at 15 times earnings is not so much.

However, other factors could play a part, as certain industries are favoured over others, risk, projecting earnings growth, etc.

As such, you can make the argument that stocks are overvalued because they're trading at all time or near all time highs in terms of price to earnings ratios and other metrics. And especially so now given that projected future earnings will have dropped considerably while stock valuations have not.