Recently I made a belgian strong ale. In the secondary I added 7 pounds of fresh cherries that were pitted, frozen, thawed, and blended up. Is there a good way to at least estimate the additional alcohol that they produced?
For fruit, get a refractometer. $50 or so online at williamsbrewing.com or other beer/wine shops. Mash up a handful of the fruit 'til it's juice and then dribble the juice on the refractometer lens. That will give you a brix reading. There's probably some formal math you can use to get exact numbers here, but here's some basic info: 24 brix when fermented dry will give you roughly 14% alcohol +/- 1% depending on just how dry it goes. So then it's some fairly basic math based on the amount of this juice you're using and the gravity of your brew.
You can achieve basically the same thing with a triple beam hydrometer that shows brix, but you'll need to mash up more of the fruit and I've found refractometer readings to be more accurate with pulpy fruit. Then again, more fruit will give you a more accurate reading because some cherries might be 15 brix, some might be 18 brix so you want a good "field sample".
For an estimate try this... Look up the nutritional values for cherries on a USDA or other calorie counting website. It will tell you the sugars per serving. From there you can estimate the weight of the sugar per unit serving weight and then get a ballpark idea.
I'm telling you now though, its significantly less than most other things we add like pure sugar or extracts. To the point where its probably not worth calculating. Especially, from an ABV standpoint. It'll change your ABV fractionally. That's from roughly calculating it in the past somewhere else.
The only thing I can think of is putting some cherries (in whatever form you would use the in your beer) in water, leaving them for some time (could be your fermentation time) and then checking the water's gravity to see how much sugar has been extracted.
Press some of the fruit to extract the juice and measure the SG with your hydrometer. Measure the SG of your beer. Then measure the volume change when you add the fruit to your beer and you'll have to determine the dilution of the sugar throughout your batch to figure out the actual increase in SG of the whole batch.