How can I properly test for signal noise?

When I'm testing audio equipment (in this case namely cables and input/output methods), how can I accurately test how much noise there is in the signal/line? (I use the two terms here more or less interchangeably) I can watch a movie and get a sense, but if I hear noise I have a hard time telling if that's from the signal or if it's originating from the media itself, in which case it's inevitable. Same goes for testing with music. Please advise.