The best test of image permanence is history. In other words, what is the oldest "image" we know of and how was it done? Unfortunately, photography is just not very old. Finding crisp, unfaded photos from 150 years ago is more a matter of luck that methodology. And color photographs? They'll fade in 20 years or less.
But there are images that have survived thousands of years; coal drawings on cave walls, pigments on ancient papyrus, etc. This doesn't really apply to photography, though . . or does it?
Up until about ten years ago the consensus was that the highest level of photographic print permanence could be achieved best via the following method:
Use quality black and white fiber-based silver emulsion printing paper, processed through a slow and precise procedure involving multiple baths of chemicals, then long wash times to remove those chemicals, then even more harsh chemicals to stabilize the remaining silver in the print to slow oxidation, and then more washing. Oh yeah, fun stuff. After all that, the print could be counted on to last a few hundred years before decay and oxidation finally caught up with it.
Then along came digital imaging technology and inkjet printers. For the most part, prints made with early inkjet printers were no better (often worse) than the cheapest color prints from your neighborhood photo lab in terms of image permanence. But a group of dedicated photographers began experimenting with different materials to stretch the technology far beyond what anyone at the time thought possible. One such photographer, Paul Roark, started pitching ideas to third party ink manufacturers to create ink sets for inkjet printers that replaced each color in the printer with a pure carbon pigment. Carbon is about as permanent as you can get. It just doesn't fade unless exposed to high temperature. And by thinning it with demineralized water it can achieve different tones.
When printed on quality, acid-free cotton bond paper you get an image that is literally as good as a cave drawing and maybe even better than the Dead Sea Scrolls!
The future of image permanence is found in the past.