Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

Stocks haven't moved much over the past couple of weeks. This afternoon, with the Dow Jones Industrials (DJINDICES:^DJI) down a meager 15 points as of 12:30 p.m. EST, stocks are simply taking a breather after their recent record run-up. Yet while broad-market followers look for signs of general economic strength or weakness, Wal-Mart (NYSE:WMT) will give a key reading on the state of the retail sector when it issues its quarterly earnings report tomorrow. With so many consumer-goods companies relying on Wal-Mart for a substantial portion of their overall sales, investors should look closely at what the retail giant has to say.

Wal-Mart will release its results before the market opens tomorrow. It typically issues a press release detailing earnings results, as well as a pre-recorded phone call in which management reviews the numbers, at about 7 a.m. EST.

Wal-Mart's challenge is that it faces competition on two major fronts. Most investors have focused on the impact of Amazon.com (NASDAQ:AMZN) and online shopping on the big-box retail industry, and Amazon's recent quarterly results showed that it has continued to make an ever-larger impression on shoppers. With Internet commerce accounting for less than a tenth of all retail sales, Amazon has a lot more room to encroach on Wal-Mart and its fellow big-box retailers.

But many investors have played down the size of the competitive threat to Wal-Mart from other big-box retailers. Both Target (NYSE:TGT) and Costco Wholesale (NASDAQ:COST) have made greater inroads toward finding steeper growth trajectories than Wal-Mart. For Costco, the ability to charge highly profitable membership fees has freed the warehouse retailer to slice margins to the bone on the goods it sells, giving customers a value proposition that has led to extremely high renewal rates that create a reliable stream of pure profit for the company. Meanwhile, Target has sought to make itself a more attractive and upscale destination for shoppers, incorporating margin-boosting measures like brand-name designer partnerships to woo customers and create buzz.

Even with sluggish results lately, Wal-Mart remains a key distribution outlet for many consumer-product giants, including several Dow components. The news that Wal-Mart gives investors tomorrow will likely send ripples throughout the Dow, especially if its results point to a weak holiday season that could have disastrous effects on retailers and product manufacturers throughout the industry.

Fool contributor Dan Caplinger has no position in any stocks mentioned. You can follow him on Twitter @DanCaplinger. The Motley Fool recommends Amazon.com and Costco Wholesale. The Motley Fool owns shares of Amazon.com and Costco Wholesale. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.