I recently bought a used 98 Ford Expedition - how can I tell if I have the MACH audio system installed?

It is not the Eddie Bauer edition, so I ume it is an XLT. It does have the factory-installed option for the 6-CD changer. I don't see the term MACH anywhere in the doentation or on the equipment, but the speakers are etched with the word "Premium". Any help would be appreciated.