Cackalak Han
Explorer
Most online ads around here list the VIN number, and you can quickly get a copy of the original window sticker from Ford's website somewhere. A little google work will find you a link to the look-up page, they tend to move it around some. I spent months comparing window stickers on used trucks before buying.
Also, all the Ford dealerships in this area have links in their ads to the original window stickers, even on used trucks, which makes it pretty simple if that's where you're looking.
At any rate, the window sticker is the quickest way to find the large tank without crawling under the truck with a tape. On my '13 there's no physical evidence of it otherwise.
You used to be able to get the original window sticker or the detailed spec sheet on etis.ford.com However, it's not worked for the last 2 years.