[Gareth]

As already said... it is possible to tell, but like Bop says, usually if you know what to look for you also know how to prevent unauthorised access (unless we're talking about Fon users, which is gonna be the exception to the rule).

Just for info, connecting to a network without authorisation itself isn't committing a crime, it's only when you use that network that you're guilty of committing a crime (usually you'd get charged with unlawfully obtaining a service)