To be sure their work runs straight and true, people use levels and squares. But after some years and much abuse, levels aren't always so level and squares aren't so true -- especially cheap ones. Should you throw them away and go out and buy new ones? Not necessarily. Today, you'll learn how to check your tools to ensure they're doing what they're supposed to. To test a level -- old or new -- put it on a flat surface, shim up one end until it reads "level," then turn it 180 degrees and read it again. If it isn't identical, it's out of whack. Same goes for a square. Draw a line with it, flip it over, then draw another on top of that. If the lines run apart, your square isn't square. For more tips, visit our Web site at onthehouse.com. And that's the On The House tip for today.