WASHINGTON—No more wondering where your hamburger came from, or where your lettuce and tomatoes were grown: Starting this week, shoppers will see more foods labeled with the country of origin.

It’s a law years in the making but timely, as China’s milk scandal and the recent salmonella-tainted Mexican peppers prompt growing concern over the safety of imported foods.

Still, hold the import-bashing: Numerous outbreaks in recent years have come from U.S.-produced foods, like spinach grown in California.

Until now, shoppers have had little clue where many everyday foods—meats, fresh fruits and vegetables, certain nuts—originate. That’s what the so-called COOL law, for country-of-origin labeling, changes.

Those who want to buy local—or who prefer, say, Chilean grapes and New Zealand lamb—can more easily exercise their purchasing power. Those worried about lax safety regulations in certain countries can avoid those imports. And the next time tomatoes are suspected of food poisoning, consumers may be able to specify they bought only ones grown in a certain region, speeding the probe.

“We do see it as an important step on the road to a more comprehensive system for tracing food items” during outbreaks, says Caroline Smith DeWaal of the Center for Science in the Public Interest.

“It will be a very good thing because we’ll have a lot more information,” adds Jean Halloran of Consumers Union. But, “you can still be fooled by the COOL label.”

How? There are bunches of exceptions. Fresh strawberries get a label but not chocolate-covered ones. Raw peanuts? Label. Roasted ones? No label. Those popular pre-washed salad mixes? Sometimes.

The new law requires that retailers notify customers of the country of origin—including the U.S.—of raw beef, veal, lamb, pork, chicken, goat, wild and farm-raised fish and shellfish, fresh or frozen fruits and vegetables, peanuts, pecans, macadamia nuts and whole ginseng. (The aim was big agricultural commodities; ginseng was added for fear of imports masquerading as U.S.-grown.)