Some lenses are razor sharp, while other lenses always produce pictures that are a bit blurry—even when the subject is in focus. As a general rule, the more expensive, heavier, and larger the lens, the sharper it will be. To get a more objective comparison of the sharpness between two lenses you’re considering, just search the Internet—the photography community examines the sharpness of all new lenses.
Tip: Prime lenses are usually sharper (and lighter, and cheaper) than zoom lenses. However, many professional zoom lenses are as sharp as the best primes. Either way, I prefer the flexibility of zoom lenses.
Please don’t obsess about lens sharpness, though. Most lenses are just fine, and your technique is much more important than the lens’ optical quality. Read Chapters 4 and 5 of Stunning Digital Photography to improve the sharpness of your images without buying a more expensive lens.