You don't even need to go to that effort. Tossing aside the fact that positive transparencies were never used in the comic book web printing process in the U.S., you can easily determine if a transparency is a proper photographic transparency. If it was printed on a water-based inkjet like an Epson, it is water soluble. On the printed side, run a wet q-tip along the edge of a crop mark where a little bit of damage wouldn't matter. If it smears, it's not a photo-based transparency. If it was printed on a eco- or bio-solvent printer like a Mutoh, you could do the same thing with a little alcohol on the q-tip, as water wouldn't affect those inks, but those printers are more expensive and less likely to be used for these purposes.