The gate behind the King Crab tavern is the only one not closed by default. All the other gates have a warning. For the south west gate, by the harbour, there are bodies all over the place when you leave, huge footprints, a comment from your main characters and a huge non-hostile orc, who can mention calling other orcs if you talk to him.
You can't necessarily tell all the time in advance what level or difficulty a group of opponents will be, but you do often see opponents in advance, and can scout ahead, or at least judge based on enemy number and composition.
