There's this article in Outlook Money:
Has Your Stock Gone Over The Hill?
It lists some "rules"...
The stock is at or above it's fundamental value. (Is this the same as
intrinsic value?)
The stock is at the high-end of its P/E range.
The earnings yield is less than that of a bond (post-tax).
Appreciation of the stock's price has made the portfolio lop-sided.
Other sell indicators mentioned are overall euphoria, extraordinary events.