On a steel frame the rust issue can be mostly internal. Outside rust starts when the paint is damaged. But if a frame is ridden in bad weather a lot the internal rust working it's way out towards the paint from the inside is a far more serious problem. Joints on the old brazed lug frames are by far the worst place to look for rust damage as the lug itself adds thickness to that area and the brazing can also support the paint from showing signs of internal rust.

Nope, truly the only way to tell if things are too crusty inside is to look inside. As you're going to or alrealy have the fork out look in the head tube and down the holes into the frame tubes as best you can with a flashlight. But the big one is to pull the bottom bracket and seat post and look through those for signs of major errosion of the steel.