Free public hotspots are usually include a "HotSpot" in it's label.

Other than that, most wifi routers of individuals are password protected using WEP, WPA, etc. so unless you actively use a hacking tool on the NIT, you won't be able to access them.

Any private wifi routers that are not protected could be considered fair game IMO. Either the owner wants to share the signal or doesn't know how to implement basic security.