I'd say you'd just have to trust them. If they know how to go about getting every bit and piece for a site they should have built it themselves in the first place.
If your website uses external css, js, php classes, includes etc it'll make it that much harder for them to find everything, unless of course they know what they are doing.
I suppose the alternative would be to show them screen shots or if you can possibly demonstrate it to them one on one using your own laptop.