Kirk Spano at Marketwatch (link) has an article about the fear of missing out. Some symptoms are checking your account much more frequently, considering much more volatile stocks or instruments. After a 150% rally off the stock market lows in March 2009 (in round numbers SPY 67 to 150), and a strong start to 2013, more and more folks are infected.
That said, some other talking head on TV pointed out that virtually no one at the coffee shop is bragging about their stock market profits or talking about their high flying winners. So it hasn't reached epic proportions that might signal a major top.
Again, I find that anecdotal stories can be powerful sentiment indicators. At the March 2009 lows, someone told me they were selling all their stocks, the exact week of the lows. At the height of the Internet bubble one of the little old ladies at church bragged to me about opening up a brokerage account for the first time in her life. During the run up in silver when it went from $14 to $49, several novices seemed to think they could not lose by being long silver.
Of course, it is easy in hindsight to see these turning points. It is not so easy in real time. It is sometimes difficult to distinguish a smart player from the classic dumb money, and rarely do they signal the exact market turn. At market tops there are a relative maximum number of buyers, at market bottoms the opposite, a relative maximum number of sellers. Nothing can change that, it is how markets work.
There are thing an investor can do to guard against the disaster moves. Avoid the all in, all out mentality. I tried very hard to tell the person that wanted to sell at the 2009 lows to scale out, to sell 20% now and then maybe another 20% in a few months. But they were too scared to consider anything other than their fear. At market tops, the opposite, there are a hundred reasons (usually all fundamentals) about why whatever they are buying will continue to go up.
Scaling in, scaling out is a reasonable strategy. I came into 2013 way underinvested in my trading account. I scaled in, day by day adding one position at a time. This way, if the market did turn, I had a variety of prices.
Another thing is to look at the chart. Runaway markets often form a parabolic blow off top. Markets don't always peak like that, but when a chart looks like a rocket taking off, ala silver running to $49, the risk for longs (and shorts for that matter) is high.
A person can learn by listening, by watching. Not the talking heads on TV which is 80% noise, but ordinary people that have always invested in something or always avoided something. When those folks move and want to talk about it, it might be useful information.