You won't see them at all unless you are currently switched to the class in question. And you'll only see them on the mini-map if you have its level one skill active (at level two, you get a trait to automatically activate that skill whenever you switch to the class).

Nodes are in clusters around certain areas. Your gathering log for each class can give you a rough idea of where to go for each level. look at the individual items, and they'll give the level of the nodes you'll find that item in, and a general location for it.