Dull knives are flat out dangerous, and it causes more kitchen injuries than sharp ones do. To determine if its sharp, the folks from America's Test Kitchen suggest grabbing a standard sheet of paper and try to slice through it. Check out the video below:

If your kinves fail the paper test you can re-align the edge of the blade so it cuts more efficiently so you won't have all that unnecessary resistance when you try to slice stuff like fruits or vegetables. A sharp blade is important because it will go where you want it go and won't behave unpredictably.

Go ahead. Try it out.

[YouTube via America's Test Kitchen (Google+)]