Clark,
Usually free wifi in businesses are safe, as they advertise it and allow you to use it (such as starbucks). Most other unsecured wifi is safe, but there is no way to be sure. However, I am sure if no one really wants you driving up and stealing their wifi bandwidth unless they agree to it first. You may just have to find businesses that allow it. They usually have passwords that you have to request.
I am sure it is safe to use more than 95% of all unsecured Wifi networks, but there is always a risk, just as there is always the risk of downloading a virus to your computer when using the internet.