When the market is on a strong bull run, many investors are lulled into a sense of complacency, believing that upward momentum will continue indefinitely. And during a major bear market, fearful investors fall prey to thinking the downward trend won’t reverse any time soon. But in reality, the market is a manic beast that can shift direction at any time. The key is to have at least some advance warning of an imminent change in trend.

There are two momentum metrics that can act as early-warning systems, both of which are readily available in popular financial publications such as The Wall Street Journal or investing websites such as Google Finance (www.google.com/finance).

The advance-decline ratio is one of the best tools for measuring the strength of the market’s trend because it examines the full market of stocks, as opposed to relying on capitalization-weighted indexes such as the S&P 500. While the S&P 500’s movement is driven by the largest names in the index, the advance-decline ratio provides a bigger picture view of the market by comparing the number of stocks that have rising prices against those that are declining.

As such, this ratio is an excellent tool for determining when a bull or bear run might be losing steam. For instance, if an index’s value is rising, but there are more declining stocks than advancing ones, that suggests the index is losing momentum and the trend could soon shift. But if there are more rising stocks than declining stocks, the trend should remain intact. The inverse is true during bearish periods.

Another handy tool for measuring momentum is the relative strength index (RSI), which compares the number of days in a given period in which an index or stock finishes up or down. An RSI reading can range between 0 and 100.

RSI is a useful tool because investors can easily customize the time period being measured to suit their investment profile; traders employ short-term RSI to inform their decision-making, while buy-and-hold investors use long-term RSI. A long-term RSI produces fewer buy and sell signals, while a short-term RSI can produce numerous buy and sell signals.

Using a 14-period RSI on a one-year chart of the S&P 500 as an example, when the RSI gets above 70 the index can be considered overbought and ripe for a downside move, at which point investors should consider selling. When the RSI hits 30, the index is oversold and poised to make a move to the upside, so investors should consider buying.

RSI can be applied to any index or stock for any period, which makes it an extremely versatile tool. But investors interested in using it should monitor the signals for a while before applying it to their portfolio, as it can take time to get a feel for what levels constitute overbought or oversold readings for a particular security.

It’s important to remember that neither gauge is entirely foolproof. From time to time, they will give false readings, so investors should always use them in conjunction with the array of indicators that comprises their analytical toolbox. But more often than not, these indicators act as a timely early-warning system, which should allow you to get ahead of a trend and position your portfolio accordingly. After all, preserving capital by avoiding major losses is one of the keys to investment survival.