Strawberries vary greatly with their ripeness and growing conditions.
If you have a refractometer, you could probably use it to get a sense of the sugar content, as that is what fruit growers use to determine when to pick the crop.
That said, if you are planning to put enough strawberries in a brew to affect the abc, you will probably end up with an extremely flavored brew.
You might want to wait till fermentation is over and then add them to stop the flavor/aroma from being driven off by the CO2 during fermentation.