[neobop]

Here are instructions for calibrating an engineer's straight level.  These levels are adjustable, but with a little ingenuity you could apply these instructions for most any level.
http://www.leveldevelopments.com/wp/wp-content/uploads/documents/Engineers-Level-Calibration-Instructions.pdf

neo

[Devil Doc]

The problem here is you need something that is flat. Not an inexpensive item to acquire. The granite ones that machinists use aren't cheap and a 1/4" piece of tempered glass isn't quite adequate. The best soulution is to befriend a machinist and use his stuff :
Doc

[Hank]

Here's a 9" x 12" x 2" granite surface plate for $41.35:  http://www.amazon.com/gp/product/B0000DD0KE/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687622&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B0035Y7OPC&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1Q52GT12SW6BAYWQVP0V

[Devil Doc]

Starrett's granite surface plates start at around $210. I wonder about the price discrepancy. Oh wait, Grizzly ain't exactly an industry standard.

Doc

[neobop]

Example 2 in the link tells you how to adjust the level even if the surface isn't level.

If you don't have a calibrated level you can still get it almost perfect.   Place the level on the surface and note the amount it's not level.  Turn the level 180° and see how the reading compares.  Adjust the level so you get the same reading both ways.  You can use thin shims under one end, or with some levels you can file down one end.   When you're checking the level make sure you put it in exactly the same place. 

Epoxy putty is perfect for building up one end.  It is easily filed or sanded so you can get it just right.
neo

[Hank]

Doc, Starrett plates are hideously expensive.  Grizzly plates are fine - take time to look at the specs.