I wrote this answer on another forum and thought I would share it with you too:
The question was "can you test a granite table with a level?"
I use a level all the time to test granite and cast iron plates. I have 2 levels, a Starrett 199 with a .0005/ 12" bubble and a King-Way 12" level with a .0003"/ 12" accuracy bubble.
I have been site testing plates with levels for 40 years and my Dad did it that way for 40 years before me.
I specialize in scraping Jig Bores and super precision machines. I recently scraped 2 Drake USA), 1 Studer (Swiss) and 1 Shigiya (Japanese) CNC Cylindrical grinders. In Final tests we ground test bars to straightness less then a .0001", one of the Drakes was .00002" before the off-sets were done. I used their Grade B plate to scrape my cast iron straight-edge. I had checked with my level and found an area that was flat to .00005" and used that area to blue up my straight-edge. My point being You can use a level to check a plate! There are better ways, but not necessary.
It's a good way to find "local errors" on a plate and best place to check my cast iron straight-edges when I am at a job-site scraping a machine. The way I do it is a lay out the plate using a straight-edge and pencil. I draw straight lines on the plate 1/2 the length of the level I am using. Cross hatching it.
I level the plate both directions before I test the plate in the center of the plate. I also check to be sure the plate is sitting on 3 points. They need to be placed in the exact position they were when it was lapped at the factory. Some plate manufactures use 25% and others use 30% on the 2 point end and in the center on the single point end. There is rubber pads or a X where they had mounted the pad. I check the levels flatness and test it's positioning error by flipping it end to end and pushing down on the ends to be sure the level is not high in the middle.
I get a pad of 1/4" graph paper and layout the plate on the paper. Then start on one end and watch the level, mark it on the paper and move the level 1/2 its length, press down on the ends, wait 10 seconds and record my measurement, move down 6" and do it again, etc. and check it again all the way down the pencil line, then do all the lines and the cross lines. I also do diagonal corner to corner check too.
I just had a surface plate lapper come to one of my scraping seminars ( the hosts friend) and demonstrate how to test and lap a granite plate. He used set of electronic levels and a repeat-o-meter.
The level method is what the used before electronic levels were invented.
Watch the Swiss Wyler video on there site and they state this in the demo. Fred V. Fowler Company, Inc. - Precision Tools and Measuring Instruments This site has a video called Passions for Precision and in the show, there is a Swiss scraper hand pull scraping the bottom of a level. I normally push scrape, but can pull scrape
Repeat Reading Gage This is the Starrett web-site. and they show a gage called a Repeat O Meter.
As I said. Before Electronic levels were invented we use bubble levels. If your working in a laboratory doing to .00002" you my want to use a electronic level or one of the above products.
If you checking your shop grade A or B plate a level will do.