What are the signs that an investment sector has reached its peak, perhaps time to get out of it?

[Dan Brewer]

Hi everyone.  Mark Twain is famous for many quotes, and one of my favorites is something similar to "Good decisions come from wisdom, and wisdom comes from bad decisions".  Essentially, we should learn from our mistakes, otherwise we are bound to repeat them.  If you have been in real estate investing long enough, you have seen both up cycles and down cycles.  It's clearly best to identify the impending peak of a cycle before it hits, otherwise you take a hit.  2007 taught me some valuable lessons, and because of that I am a much better investor today.  

So for the benefit of others as well as ourselves, what are the signs investors should look for that tells us the market is maturing/has matured, and how should we as investors react?  It may vary depending on what real estate sector we are in, but there are probably some pretty common signs we all should look for.  Are we at or near a peak?  

Thanks for your insights.

Dan