To burn correctly, grain must be dry. Check the document Grain Moisture for Proper Combustion for more details.
Buying corn from a commercial establishment such as a feedmill or grain elevator they will have grain moisture testers and readily be able to tell you the moisture content of the grain you are buying. Many farmers also have grain testers -- though you will find some of the older ones can tell you within half a percent or so how dry the corn is just by sticking their hand into it.
If you are buying from a smaller farmer who doesn't have a tester, and you don't quite trust his guess-work, most grain elevators will be willing to test a sample for you. Just bring them a coffee can sized sample of the grain you are thinking of buying. They will test it and give it back to you.
There are also options for you if you would like to do your own testing. For instance, a digital grain tester.
Gemplers $235.00 Direct digital readout for corn, wheat and soybeans. Site: http://www.gemplers.com/a/shop/product.asp?T1=109819&UID=200606201312513903400610
Using a Microwave Oven to Determine Moisture in Forages
The second option involves more work but allows you to perform your own grain moisture test using an accurate scale and a microwave oven to super dry the corn. An article by, Barry Steevens, Ron Belyea and Richard Crawford from the Department of Animal Sciences at the university of Missouri describes the proccess in detail.
What is the relationship between grain moisture and test weight?
Grain moisture and test weight are related from the standpoint that as moisture increases, test weight decreases. For example, corn at 20% moisture will have a test weight that is 2 pounds lower than the same corn dried to 15.5% moisture. It doesn’t matter whether the drying is done naturally in the field or artificially in a bin. This year’s high measured test weights may be due, in part, to the fact that corn was much drier coming off the field than is normally the case. One reason why test weight increases as grain dries is that dry kernels pack together more easily than wet ones. Additionally, as moisture decreases, the kernels shrink and this allows for more kernels to fill a volume bushel.http://www.uwex.edu/ces/crops/BushelsTestWeight.htm
Test weight usually increases as grain dries, but the exact relationship between moisture and test weight depends on drying conditions and grain characteristics and is difficult to predict.
Grain is seldom sold at the standard moisture content (corn = 15.5%). When grain moisture content is greater than the standard, the grain weight is discounted to account for the extra moisture according to the formula: (100%- wet %) divided by (100%-dry %). For a sample of soybeans at 18 percent moisture, the calculation would be: (100-1Cool divided by (100-13) = 82/87 = 0.94. Multiplying the weight of the wet grain by 0.94 will give the weight of the grain at 13 percent moisture. For example: 6000 pounds of soybeans at 18% grain moisture would become 5640 pounds of grain at 13% moisture (6000 X 0.94 = 5640).
If grain is dryer than the standard, that same equation can be used to calculate the increased weight that should be credited the seller although that calculation is seldom made and the seller is not usually rewarded for the low moisture content. This calculation works for any grade of any grain for which a standard moisture content is specified. http://ohioline.osu.edu/agf-fact/0503.html