When the database contents are re-bound to the grid view from the database, they are always Strings. Even if you have a numeric or date column, the data is rebound as a String.

so i think there is no method to discovering the datatypes of your database table in gridview.

but you can check on label and bind all the column with datatypes.

try this link 

http://asp-net-example.blogspot.in/2011/04/how-to-get-datacolumn-datatype-from.html