The hotspot's name (SSID) is a good place to start. If it's named something like "Anytown Public Library", it's a good bet it's free. If not, you'll know as soon as you connect because it'll ask you to log in. If the network name is something like "SpongeBob Square Pants" (to use an example of someone's network in my apartment complex), it's probably a private network. It may be unsecured, but the legality of using it without permission is ambiguous at best.