They'll usually place a sticker on the unit that indicates that it has the package installed.
If it doesn't have that look at the windows and see if there's a mark that indicates they're thermal pane. Also look at the furnace and see if it's a 40k BTUH unit. If both those things are in there, it's most likely all installed.
You can also call Sierra division and give them your VIN and they should be able to tell you if it was installed.
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!