The concept of "overvalued" assumes a valuation model that everyone agrees on. Large companies are complex organisms and equally complex to value. It follows, with ample evidence, that simple reductionist valuation models based on P/E, book value, and other easy-to-obtain numbers are going to be inadequate. And most valuation models are terrible at incorporating myriad risks.

Currently, I would argue that some large caps are clearly overvalued but others are also clearly undervalued, based on my own valuation model. The indexes are almost always a mixed bag but I buy individual large caps so that doesn't concern me much. It is rare for the entire market to become overvalued, in which case the smart move is not to buy in.