Dividend investing is a tried-and-true strategy for generating strong, steady returns in economies both good and bad. But as corporate America's slew of dividend cuts and suspensions over the past few years has demonstrated, it's not enough simply to buy a high yield. You also need to make sure those payouts are sustainable.

In the latest edition of this series, let's examine how United Parcel Service (NYSE: UPS) stacks up in four critical areas to determine whether it's a dividend dynamo or a disaster in the making.

1. Yield
First and foremost, dividend investors like a large forward yield. But if a yield gets too high, it may reflect investors' doubts about the payout's sustainability. If investors had confidence in the stock, they'd be buying it, driving up the share price and shrinking the yield.

UPS yields 3%, a bit better than the S&P 500's 2.2%.

2. Payout ratio
The payout ratio might be the most important metric for judging dividend sustainability. It compares the amount of money a company paid out in dividends last year to the earnings it generated. A ratio that's too high -- say, greater than 80% of earnings -- indicates that the company may be stretching to make payouts it can't afford, even when its dividend yield doesn't seem particularly high.

UPS's payout ratio is a moderate 48%.

3. Balance sheet
The best dividend payers have the financial fortitude to fund growth and respond to whatever the economy and competitors throw at them. The interest coverage ratio indicates whether a company is having trouble meeting its interest payments -- any ratio less than 5 is a warning sign. Meanwhile, the debt-to-equity ratio is a good measure of a company's total debt burden.

UPS' debt-to-equity ratio is 146%. That's considerably higher than that of its peers', but the interest it owes is covered a full 18 times over.

4. Growth
A large dividend is nice; a large growing dividend is even better. To support a growing dividend, we also want to see earnings growth.

Company

5-Year Earnings-per-Share Growth

5-Year Dividend Growth

UPS 2% 7%
FedEx (NYSE: FDX) (5%) 8%
Union Pacific (NYSE: UNP) 19% 21%
Norfolk Southern (NYSE: NSC) 8% 21%

Source: S&P Capital IQ.

UPS has struggled to grow its earnings over the past five years, in an environment which apparently was much more forgiving to rail than air freight.

The Foolish bottom line
UPS exhibits a fairly clean dividend bill of health. Generating additional earnings growth would be beneficial, though the moderate payout suggests it could be possible to continue growing the dividend at a faster pace than earnings. To stay up-to-speed on UPS's progress, or on any other stock, add it to your stock watchlist. If you don't have one yet, you can create a free, personalized watchlist of your favorite stocks by clicking here.