Jim Cramer's Portfolios of the Week

PEWAUKEE, Wis.(Stockpickr) -- According to Jim Cramer, investors need to know the four enemies of all bull markets. On Wednesday's "Mad Money" TV show, Cramer reminded viewers that all bull markets must come to an end. However, he said the key to investing successfully is to sell your stocks before that happens.

With this in mind, he pointed out some key things for investors to look for so they can avoid any looming selloff. One warning sign that investors should look for is inflation. Cramer said inflation has killed almost every bull market since he started his career on Wall Street. He explained that inflation usually means the Fed will raise rates, which can hurt earnings and force investors into high-yielding securities over stocks.

Another problem to watch for is "super extreme valuations" in stocks. Cramer reminded viewers that stocks were so overvalued in 1987 and 2000 that earnings per share were put on the back burner to sales per share, takeover potential and future earnings. Cramer said if you see this, you have to get out of stocks because you will get slaughtered.

Cramer said another issue that can lead to the end of a bull run is if the market loses its leadership stocks. He mentioned that in 2007 and 2008, one leader after another started to collapse and trade lower. Cramer said when this happens the followers begin to sell off, so investors have to pay attention to the action in market leaders.

Finally, Cramer told viewers to watch out for high unemployment. He said jobs are key to any bull market because two-thirds of the U.S. economy is a consumer economy, so without job growth we can't sustain a bull market. Cramer warned investors that a jobless rate over 10% will stop a market rally cold.