My initial reaction was "what idiot did you reach at Toyota?"... but then, maybe this was at a dealership, where I have learned to have low expectations.
More useful answer: it depends on the engine, and thus the year.
The transmission oil cooler included in the package for the first generation (1998-2003) is very visible, and photos were posted at some point... but I believe that lexi has a 2005 Sienna.
The topic Sienna Gen-II Towing Capacity.
(which is very long) explains the package equipment for the early and late second-generation versions, and shows (in photos) exactly what to look for in the later years (2007-2010); unfortunately, the difference in early years (2004-2006) is not readily visible.
I called Toyota Canada and they explained that all Canadian-market Siennas (at least in my year) had the package, so that was easy. I would hope that Toyota USA customer service or a dealer could check factory equipment if given the VIN for reference; perhaps I am overly optimistic.
While it's tough for us to identify the difference for 2005, for the people with all the part numbers and production information it would seem to be an easy lookup. It should appear on the sheet originally pasted in the window anyway, since it is (in the U.S.) an option.