An S will have the basic specification audio system, unless it was up-specced at the time of ordering.
You can tell pretty quickly, simply by looking.
The sub is under the boot floor shaped to curve round the the right hand side of the spare wheel. The speaker grills will also have Alpine badges, and the stereo has the CD slot above the screen and 12 buttons below the screen, whereas the basic stereo has the CD slot below the screen, and 6 buttons below that.
Hope this helps.
So are you looking for a FL2?
The SE or SX have the best specs, without going to the additional cost of the HSE. There's not much difference between the SE and HSE, except for 3 major differences, all of which can be retrofitted.
The XS is pretty much the same as the SE, although has full leather, but missing the sun roofs and often the nav system too.