Title says it all: How do I determine the alcohol content of a mixed-drink (Cocktail, Longdrink)?
Note - I know the alcohol content of all ingredients.
To a first approximation, you can just calculate the volume of pure ethanol in each of the alcoholic ingredients (total volume * %ABV = volume of pure ethanol), add those up, and divide by the total volume of all ingredients.
For example, a margarita with 35mL 40% ABV tequila, 20mL 40% ABV Cointreau, and 15mL lime juice would have alcohol content of (35mL*40% + 20mL*40% + 15mL*0%)/(35mL+20mL+15mL) = 31% ABV.
That's an approximation because the volumes are not strictly additive: mixing 50 ml pure ethanol with 50 ml water will actually give you something like 95 ml of the mixture. To get a truly accurate answer, you'll need to measure the volume of the final drink, and divide your total alcohol content by that.