This can only become a systemtic concern if the growth of money and velocity exceeds GDP growth. In such a case, you would expect inflation (or huge asset bubbles) and relatively there is little in the US.

Real estate prices are well in historical norms compared to rents (http://www.economist.com/blogs/graphicdetail/2015/11/daily-c...)

By what measure are stocks overbought? If we blindly look at earnings, stocks have an earnings yield of about 3% over a 10 year Treasury note, well within accepted ranges (http://www.investopedia.com/terms/e/equityriskpremium.asp)