I do my best to go into the local schools on their CAREER DAYS and educate the jr. high kids about the music publishing industry. I give them a handout with a FAQ about copyright. Education should start early, but I'm here to tell you as a personal example that one can get all the way through music school and not be educated - exc. "backhandedly" - about copyright and its implications and how to deal with it.
Given the way that copyright law has been bastardized by congresscritters bought and paid for by Disney et. al, that's just about right.
Better rule of thumb, is anything prior to about 1930 is public domain, stuff produced afterwards is difficult, if not impossible to determine without quite a bit of research.
For myself, if the work is more than 30 years old, it's fair game. 28 years was long enough copyright for Jefferson, it should be for us as well.