Open developer tools and type `document.lastModified` in the console. Though this is painful for every web page, is useful in these instances. Maybe someone with experience in developing Chrome extensions, can develop one for getting this plus other useful page info by just clicking on it in the toolbar
why would companies who are trying to deceive not go through the trouble of changing the last-modified header? At any rate lots of sites don't set last modified anyway so that makes it January 1, 1970, GMT.