The nutritional content of foods is not an exact science. When the folks that determine information like the chart below, individual foods are tested and nutritional values can be ascertained accurately. An average can be obtained by testing multiple examples of the food in question and the values in the chart determined. All this is well and good, but the actual nutritional content of the piece of fruit you put in your cart at the grocery store is very much an unknown. This fact provides additional reason to supplement your diet. Supplements do include a known quantity of nutrient. If you are interested in testing for vitamin C content, please visit our Testing for Vitamin C page.
You may also wish to refer to Vitamin C Content for Selected Fruits and Vegetables for additional information. Also, fatfree.com has an interface to the USDA Nutrient Values database of about 5,000 entries. This database is huge, but remember that the values are averages and may not exactly reflect the particular item of food you are interested in. For more information on fruits, please visit The Vitamin C Content of Fruits of the World
Please keep this in mind when attempting to evaluate the nutritional value of foods you've eaten.