It is much easier than that. You calculate it.
The package should tell you the number of calories per portion and define the portion as its weight in grams.
You will also know the number of M&Ms in a package. And how much the whole content weights (net content).

Nutritional facts for peanut M&Ms

http://www.thecaloriecounter.com/Foods/1900/19140/4/Food.aspx

But they make it easier:

Serving Size 10 pieces (20g)
Calories 103

10 pieces ------- 103 calories
1 piece -------- X= (1 p * 103 cal)/ 10 p = 10,3 cal

If you want to measure it yourself try the classic methods.
"Classically the total energy content of a food was determined by burning the food in a bomb calorimeter and measuring the amount of energy released. "

This is what they actually do: meassure the percentage of components and calculate total calories.

"The energy information on labels is determined by analysing the chemical composition of a food, the concentrations of protein, fat, carbohydrate for example, and adding the energy value of the components to give a total value. Note that methods for determining carbohydrates, including fibre, in food are still the subject of discussion and methods are subject to ongoing modification."
http://www.dairyscience.info/energy_label.asp#3

This site refers to
http://www.fao.org/DOCREP/006/Y5022E/Y5022E00.HTM

Food energy - methods of analysis and conversion factors
Report of a Technical Workshop, Rome, 3-6 December 2002

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
Rome, 2003

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