Levels are finely-tuned instruments. A lot depends on how precisely that little vial is aligned, and if you drop your level, it might throw things off. To make sure your level’s on the level, give it a test. Hold the level up against a sheet of drywall or any surface you can draw a line on. When the bubble sits at level, draw a line along the bottom edge of the level.
Now, fill a length of clear plastic tubing with water and tap out any bubbles. Hold the tubing so that the surface of the water in one end is aligned with one end of your line. The surface of the water at the tube’s other end should line up with the other end of your line.
To test the plumb vial on a level, hold it vertically and draw a line. Then test this line using a plumb bob.
Tape measures are great for work where you don’t need perfect accuracy. But you should still make sure it’s measuring properly. The tab at the end might bend, causing you to read longer than the actual measurement. As well, the tab should slide easily back and forth. This is so you can measure two ways — by hooking it over the end of a piece of wood and by shoving it up against a surface for inside measurements — without including the thickness of the tab in your measurement. Test this by using a straight rule on a piece of stock, marking off at 1′ increments using a utility knife. Try your tape measure, both by hooking it over the end of the stock, and by holding the stock against a flat surface and shoving the tape measure up against it.