Although I expect the market to take a breather and potentially correct during the first quarter, I do not believe that stocks are in bubble territory. The first chart shows the long term S&P500 trend with standard deviation bands above and below. By this measure stocks are not extended much above the long term log scale trend.
This second chart divides the US 10 year Treasury by the S&P500 Earnings Yield (which is 1/Price Earnings ratio). It also suggests that stock prices are not in bubble territory.