How do you find out the rated capacity of a wind turbine?

From the manufacturer. All wind turbines are specific models with a name and/or model number. Once you find that, you can go to the manufacturer's web site and get the rating.

Another tack to take is to look at the permits, these are often public documents on the permitting agency's web site. Rated capacity is an important piece of information that will be on any permit.

Just remember that the rated capacity is the maximum power output of the turbine at the most favorable conditions. Your actual generation will be a fraction of the rated capacity, usually in the 20 - 30% range.