NevOn: Hidden dangers with public Wi-Fi

NevOn: Hidden dangers with public Wi-Fi: "It's becoming a common thing to see people sitting in public places - train stations, airports, the local Starbucks - with their laptops, doing their email or writing their blogs, or any manner of things that they'd do when in the office or at home.

How do you know that the wi-fi network you've connected to really is secure?"