Warren Buffett became one of the world’s wealthiest men by finding undervalued stocks and stealthily building up large stakes in them. Many investors have sought to copy his style of investing, learning what he finds to be appealing though mandated public 13F filings with the Securities and Exchange Commission (SEC). These documents, filed within 45 days of the end of every quarter, show how many shares were bought and sold in each particular stock.

Here’s an example of Warren Buffett’s most recent 13F filing.

As the table of stocks in the document reveals, Buffett continues to hold very large stakes in companies like American Express (NYSE: AXP), Coca-Cola (NYSE: KO), and Procter & Gamble (NYSE: PG). He's held those investments for a very long time, so long-time Buffett watchers are likely to gloss over those names. Instead, they keep an eye out for changes in his investments in lesser known companies.

For example, if you compare this most recent filing with the August 2010 filing, you'll notice that Buffett's investment in Johnson & Johnson (NYSE: JNJ) has nearly doubled from 23.9 million shares to 42.6 million shares. That's a $1 billion increase. You'll also see that Buffett sold off his 2.75 million share stake in Home Depot (NYSE: HD) last quarter.

Investors can use this information in different ways. For some investors, it’s a validation of their existing moves. If Warren Buffett is loading up on a stock they own, then they're doubly convinced they're on the right path. Other investors use the information to create a short list of stocks they should watch, should the market hit a periodic slump. As prices dip, you can bet that Buffett will see even more value in the names than he did before.

But 13F filings can also be used to determine which stocks to avoid or sell. If you own, or were looking at investing in Home Depot, Buffett’s decision to exit the stock should give you pause.

It also pays to focus on fund managers that tend to take activist roles to unlock shareholder value. For example, Bill Ackman and Carl Icahn like to build investments in underperforming companies, get a few seats on the board of directors, and then agitate for change. The sheer prospect of pending cage-rattling is enough to push a stock up as investors come to expect management to respond to the activist investors. Carl Icahn recently started buying shares of Mattel (NYSE: MAT) which may be the first move in an eventual battle for the company’s value unlocking efforts.

The Smartest Guys in the Room

Virtually every major hedge fund and mutual fund manager is required to file a 13F, so you can track anyone you want, based on your investing style. Like value stocks? Take a look at the latest moves in your favorite value stock funds. Here's quick list of some of the top money managers that you may want to regularly follow:

  • Warren Buffett (Berkshire Hathaway)

  • John Paulson (Paulson & Co.)

  • David Tepper (Appaloosa Management)

  • Eric Mindich (Eton Park Capital)

  • George Soros (Quantum Fund)

  • Carl Icahn (Icahn Capital)

  • Bill Ackman (Pershing Square)

  • Ron Burkle (Yucaipa)

And many more. The best fund managers for you to follow are going to be the ones that match your investment style. Remember that these filings come 45 days after a quarter has ended, so the information in the 13F filings can be slightly out of date, which means you shouldn't simply buy a stocks just because it's a current favorite of these investment pros. Instead, use these filings as a guide for your own deeper research. If you like a stock -- and the big boys do too -- then you've likely got a winner on your hands.