Telltale signs a car has Premium Sound

[Skyline]

I'm looking to upgrade my E53 to an E70. While shopping for cars, I'm finding many sellers have no clue what option packages their cars have. I'm looking at one that's pretty far away, and remotely looking for clues as to what options the car has with a somewhat naïve owner is tricky.

I know what the premium sound consists of, but what, if any, are the visible clues that a car has this option? What's the easiest way to tell for sure so I can email this seller and ask him to check for me?