How can you tell if a phone is muted or not just by looking at its file settings?

Ok weird question but I still would like to know.

Ok I'm not rooted (well one of my phones isn't rooted, which is besides the point). If I wanted to know if a non rooted phone was muted or not just by looking at its system contents (the root directroy of an android phone) how could I tell if a phone was muted? Which file would I have to view in order to ascertain that information? E.g. System > etc > firmware or more likely build.prop