How can we tell if the U.S. stock market is getting too pricey? Well, if you watch CNBC long enough or read enough stories in the financial media, you are likely to learn dozens of ways people will try and answer that question. There is not one right answer. If there was, successful investing would be easy and it is far from it.

I decided to dig into the numbers and present one way we can evaluate the stock market at today’s levels relative to prior market peaks, in order to see if we are nearing a point where we should start to get worried. I chose five of the most noteworthy market peaks over the last 25 years or so. After each of these peaks, the S&P 500 index fell at least 20% peak-to-trough. Some of the corrections were relatively normal, mild bear markets (the 1990 recession; -20% and the 1998 Asian financial crisis; -22%). Others were more pronounced (the 1987 crash; -33% and the dot-com and housing bubble bursts of 2000 and 2007; -50% and -58%, respectively).

I have graphed the P/E ratio of the S&P 500 index at each of these five market peaks. At one extreme we have the 1990′s bull market led by internet stocks, which saw equity valuations easily reach record-high levels, but at other peaks the results are more uniform, with markets typically topping out with P/E ratios in the high teens or low 20′s.

As you can see, today the S&P 500 sits at 16x earnings. While we are approaching levels that should be considered elevated, one can argue that another 10-15% upside in P/E ratios would not be out of line with historical data. That said, making a large bet that valuations will reach the high end of the historical range is not something I would take to the bank. To me, this data says that the market is starting to get pricey, and although we could very well squeeze more upside out of this bull market (largely because with interest rates so low, equity investors are willing to pay more for stocks), I would still be cautious. As a contrarian, ever-higher stock prices only increase my preference to raise more cash and wait for the next correction, even if we don’t know exactly when it will come.

If you are thinking we have seen this level on the S&P 500 index before, you are absolutely correct. As you can see, the last 16 years or so has been a roller coaster ride, with three separate bull runs to around these levels, and the prior two have ended badly thanks to bubbles bursting (dot-coms in 2000-2001, housing in 2008-2009).  So do you want the good news first or the bad news?

The bad news is pretty evident from the chart. We have reached a triple top and U.S. stocks have now risen 125% from their lows made in March 2009. That is a huge move in just the last four years. It warrants being cautious in the short term, as the market does feel overbought here.

The good news is actually pretty good though. At the March 2000 peak of the dot-com bubble, the S&P 500 reached 1,553 and the index components earned $56 in profits. P/E ratio: 28 (the highest ever recorded). Students of market history should have realized that stocks were dramatically overvalued. (Author’s note: As a college sophomore at the time, I was less than well-versed in market history, so it was the beginning of my history lesson, and a very good one at that).

At the 2007 peak of the housing bubble, the S&P 500 once again pierced the 1,500 level, topping out at 1,576. Earnings for the index hit $88, giving the market a P/E ratio of 18. That is still a high valuation, but rather than being unprecedented, the market was simply at the top end of its historical valuation range. Dangerous, yes, but not unheard of.

You can see where I am going with this. Today the S&P 500 sits at 1,502 and 2012 earnings are likely to come in around $100. That P/E ratio (15) is only slightly above the long-term median of 14. So the U.S. stock market is not materially overvalued as it was in both 2000 (by a large margin) and in 2007 (by a smaller margin). Now, that does not mean we cannot see stock prices fall meaningfully from these levels. After all, P/E ratios aren’t everything (despite the dot-com bubble being far more dramatic in terms of overvaluation, the market actually fell more after the 2007 peak because the economic shock was larger), but U.S. corporations are now earning enough in profits to justify the S&P 500 trading at 1,500, especially compared with the two prior peaks on this long-term chart.

My takeaway: it makes sense to be cautious, but not alarmed.