The best way to do this is using a professional wind power predictor tool that measures wind speed and turbulence and allows you to predict the amount of power you can generate using wind power.
There are a number of wind turbine manufacturers around. Unfortunately, a lot of the cheaper turbines - such as the type found for sale on eBay - are quite often extremely heavy and have poor low-wind performance. The best turbines will typically start spinning in a 5km/h breeze and will start generating electricity at around 8km/h (5mph) whilst a lot of second-rate models won't start generating electricity until they get a 20km/h (12mph) wind. The additional weight of these cheap turbines mean you have to have significantly stronger mountings and the extra cost of this will often outweigh the cost savings made by buying a cheap turbine as opposed to a good one.
Wind turbines are an area where buying cheap is almost always a false economy. Buying a good make, such as Rutland, Wren or Coemi will save you money and ensure you have a reliable system in the long run.
Compared to solar, turbines are extremely location sensitive. Whereas most people can implement a small solar array in most locations and achieve success, the same cannot be said about turbines. Find the right site, and a turbine can be unbeatable. Find the wrong site, and they can be a complete waste of money and time.