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

What: Shares of equipment rental company United Rentals (NYSE: URI) were getting a big push from investors today, rising as much as 14% in intraday trading on a strong earnings report from a key competitor.

So what: Though Ashtead -- the owner of the Sunbelt and A-Plant rental brands -- is based in the U.K., it actually does most of its business in North America. That's why United Rental investors are smart to keep an eye on what's going on at its rival.

In its quarterly report, Ashtead presented a very bullish view on the equipment rental business, suggesting that there is a "structural shift" going on in the industry. While it's common for companies to rent equipment rather than buy new equipment during tough economic times, Ashtead believes that cash-strapped construction companies with less access to financing will continue to opt for renting even as times get better. If they're right about this, it would be a trend that would not only benefit Ashtead, but the sector more broadly, which is great news for the industry leader … which happens to be United Rentals.

Now what: After a few years of losses, a sharp recovery for United Rentals would be a very big breath of fresh air for investors. Because results have been depressed, the stock's trailing valuation multiples look frighteningly high, but if Ashtead's view is on target and Wall Street has a good read on United Rental's recovery potential, this stock could start to look very cheap.

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Fool contributor Matt Koppenheffer does not have a financial interest in any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or Facebook. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.