Unless you know who made the wallpaper your looking at,, just a lot of research !! If it has a name on it you might see if the company is still in business, and what years certain patterns were produced. Most of the vintage wallpaper I have seen does not have a baseball white backside, Tends to darken a bit over the years, Newer wallpaper tends to stay white longer since most of it is treated with a fixative so it won`t fade !!