A solvent agression ladder. For unknown surfaces, start at the top (least aggressive) and work toward the bottom.
Test in an inconpicuous area first, of course.
Water
soap & water
alcohol (denatured)
Naptha (lighter fluid)
Mineral spirits
Diesel fuel
Gasoline
MEK Methyl Ethel Ketone
Lacquer thinner
Acetone
Methyl Ethyl Chloride (paint stripper)
Sometimes, it helps if you know what the paint composition is, as different paints have different sensitivities to solvent attack.
Catalysed paints are usually more resistant thing down the ladder than simple solids paints.
Cheers,