Paul,
Try holding it up to the light to see if impressions in the paper show up the way a watermark would show up or try a digital camera that makes automatic lighting adjustments may reveal impressions or residue when you use ISO mode (no flash, ambient light only) and the light is low (you 'll have to experiment to find out how low to adjust your light).
If that does not work try infrared or blacklight or maybe Ultraviolet light, would probably work best in a dark room , good luck
Have a read here as well:
http://www.inkpal.com/ink-news/how-to-fix-faded-ink-on-old-documents/ Mac 66.
