If you want it to be non-destructive (well, silver prints don't smudge, for instance, when rubbed with a cloth), I think you'll have to rely on visual methods. It's not that difficult, really, if you are accustomed to looking at a lot of prints.
Another photographer once asked me to make a few prints prints for a set that he was putting together for an appraiser who gives workshops about appraising photographs. The full set was around 20 prints from the same neg, all different processes including silver on enlarging paper, Azo, pt/pt, cyanotype, digital from an office printer, Piezography by a skilled digital printer, xerox copy on OHP film mounted on a white board and matted, a silver print made from a 35mm neg exposed at the same time as the 8x10" neg used for the other processes, etc. They were mounted in a uniform way and numbered on the back. I could identify most of them, except for a few that were made with one process over another process. The digital prints were obviously digital (which is not to say that they were good or bad prints, but they didn't look anything like any wet process).
The workshop participants, who were mostly art and antiques appraisers interested in getting into photographs, could only identify 2 or 3. I think one may have gotten about 7, and this was someone who worked in the photo department of a museum or auction house, as I recall.
If you had a "litmus" test, what would you do with hybrid processes--digital negs for traditional processes, silver prints made with the DeVere digital enlarger, Lambda/Lightjet to silver gelatin or RA-4 type paper? Visual inspection would be the only way to determine the process.
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