How do you measure ABV in a Fruit Beer?

I understand the entire concept of measuring your ABV but my question is how do you measure it in a fruit beer?

So you pour the wort into the primary and you get an OG but you add puree or fruit into the primary and/or secondary, you are adding more sugars but some has already been fermented by the time you add your fruit. I hope this doesn't sound confusing, I'm just wondering how much extra ABV is added with the fruit additions.

You have to determine the fermentable sugar content of the fruit, which is generally small. I added 3 lbs of blueberries to a wheat. At 14 gm of sugars per 175 gm serving, that's less than 4 oz. I suspect that was more than balanced out by the increase in volume (which I have not tried to calculate) and the ABV dropped.

So basically it sounds like a pretty minimal increase in ABV?