At which point is equity market overheated?

[Kross]

Some people love to invest in equity markets. But there are times that I hear that the equity market is overheated.

At which point is the equity market overheated? How does a investor know that the equity market is overheated?

 

[Fidelia]

That point in a market where even if new money comes into the market, there would be no significant gains. Market is saturated at that point.