Although business headlines still tout earnings numbers, many investors have moved past net earnings as a measure of a company's economic output. That's because earnings are very often less trustworthy than cash flow, since earnings are more open to manipulation based on dubious judgment calls. Earnings' unreliability is one of the reasons Foolish investors often flip straight past the income statement to check the cash flow statement. In general, by taking a close look at the cash moving in and out of the business, you can better understand whether the last batch of earnings brought money into the company, or merely disguised a cash gusher with a pretty headline. Calling all cash flows When you are trying to buy the market's best stocks, it's worth checking up on your companies' free cash flow once a quarter or so, to see whether it bears any relationship to the net income in the headlines. That's what we do with this series. Today, we're checking in on Fair Isaac Corporation (NYSE:FICO), whose recent revenue and earnings are plotted below.

Source: S&P Capital IQ. Data is current as of last fully reported fiscal quarter. Dollar values in millions. FCF = free cash flow. FY = fiscal year. TTM = trailing 12 months.

Over the past 12 months, Fair Isaac Corporation (NYSE:FICO) generated $104.3 million cash while it booked net income of $92.0 million. That means it turned 15.4% of its revenue into FCF. That sounds pretty impressive. All cash is not equal Unfortunately, the cash flow statement isn't immune from nonsense, either. That's why it pays to take a close look at the components of cash flow from operations, to make sure that the cash flows are of high quality. What does that mean? To me, it means they need to be real and replicable in the upcoming quarters, rather than being offset by continual cash outflows that don't appear on the income statement (such as major capital expenditures). For instance, cash flow based on cash net income and adjustments for non-cash income-statement expenses (like depreciation) is generally favorable. An increase in cash flow based on stiffing your suppliers (by increasing accounts payable for the short term) or shortchanging Uncle Sam on taxes will come back to bite investors later. The same goes for decreasing accounts receivable; this is good to see, but it's ordinary in recessionary times, and you can only increase collections so much. Finally, adding stock-based compensation expense back to cash flows is questionable when a company hands out a lot of equity to employees and uses cash in later periods to buy back those shares. So how does the cash flow at Fair Isaac Corporation (NYSE:FICO) look? Take a peek at the chart below, which flags questionable cash flow sources with a red bar.

Source: S&P Capital IQ. Data is current as of last fully reported fiscal quarter. Dollar values in millions. TTM = trailing 12 months.

When I say "questionable cash flow sources," I mean items such as changes in taxes payable, tax benefits from stock options, and asset sales, among others. That's not to say that companies booking these as sources of cash flow are weak, or are engaging in any sort of wrongdoing, or that everything that comes up questionable in my graph is automatically bad news. But whenever a company is getting more than, say, 10% of its cash from operations from these dubious sources, investors ought to make sure to refer to the filings and dig in.