Relying on online consumer reviews to find a contractor or pick a blender may be dicey.

In September, the New York attorney general fined 19 companies $350,000 for writing fake online reviews for businesses. The problem is even more pervasive than that—a new study from researchers at Harvard Business School and Boston University estimates that nearly 20 percent of reviews on alone are bogus. Researchers at the University of Illinois put the number at of fake online reviews closer to 30 percent.

Review sites use filters and review teams to weed out suspicious ratings, but it’s still on consumers to give what they see a reality check. Even without fakes to contend with, a quick glance at the star rating may not tell you everything you need to make a decision.

One trick I like to use: Even for a highly rated product or service, read the worst reviews. That’ll give you some expectations of the worst you might encounter, and help you plan accordingly—whether that means spending your money elsewhere or modifying your plans (like sticking with that sweater, but getting a different color if reviewers say it’s more pink than red).

For example, 119 Zappos reviewers gave the New Balance M990 sneakers a collective rating of four stars out of five. But of the 13 percent of reviews critical of the product, most complain that a strong smell of cat urine develops after the shoes have been worn. Worth factoring in, right?

Of course, the negative reviews can be made up, too—by a ruthless competitor. So take everything with a grain of salt.

By CNBC’s Kelli B. Grant. Follow her on Twitter @kelligrant.