There often are product listings or catalogs from which the client picks their colors, shapes, models, options (such as the few colors of roofing that qualified, a triple-glazed patio slider instead of a double-glazed, etc) and a lot of times, it's on the invoice or estimate for work. The other place to look is inside the window track or edge. Not the large stickers that get removed. If you are looking at used homes, always check the window casement insides for the production labels, to see the year. The same is true of toilets; the year and model are inside the tank, typically.