Being able to tell if a stock is cheap or expensive is something very difficult. It will take you years before getting there. And even after many years, you'll still be unable to be sure about your impression. You'll mostly get to a point of approximate comfort or discomfort with a stock. That's the best you can aim, in my opinion...
An example of an expensive stock is Apple (AAPL). The current PE is 22 and the FWD PE is 18. You've surely seen scarier valuation than that...
But historically, Apple's PE is way lower than that. See for yourself:
Last time the PE was this high was 2009. Let's add that, in 2009, Apple was a much smaller company. And the Iphone/Ipod was the new revolution (there's no revolution in sight these days).
Analysts expect 10% annual growth for the next 5 years. I think it's reasonable but I don't think we could expect much more for such a giga-cap.
Apple is still a great great great company. I wouldn't tell anybody to sell an entire position. But at the current level, I think a reduction would be appropriate for someone owning a big position.
But, do what you want. I don't care. What matters to me is my own portfolio. Fuck you.