This Thing lets you efficiently check that your entire print bed is leveled across the entire surface. I keep it on my SD card so I can quickly print it before starting a print, which improves the success rate of my printing considerably.
I've attached the STL of the version that I use (Replicator, 0.1, 0.2 and 0.3 mm height.
It's customizable so you can enter your print bed size, height and width.
Use the Customizer to generate a print bed leveler for your print bed and layer height. If you have a Replicator (original) you can use the STL files attached here.
Print it. The plastic should be the same color and width across the entire print bed.
If the layer gets squished too thin and breaks up, lower that area of the print bed.
If the layer is lighter, narrower or doesn't stick to the print bed at all, raise that area of the print bed.
On the Replicator, the bed leveling screws are on the corners. So if the area to adjust is in the middle, adjust both screws on that side equally.
Repeat printing and adjusting until the print is level across the entire print bed.