1. In an inconspicuous area, clean the paint surface with a general purpose cleaner. Let dry or wipe dry.
2. Next, rub the clean surface using a clean rag. Dampen with denatured alcohol. If you see paint residue on the rag, it is most likely water-based paint.
3. After rubbing the area with denatured alcohol, feel the surface. If smooth surface and no paint residue, the the paint is oil-based. If it is slightly tacky and has paint residue, then the paint is water-based.
Latex paints are made from a water-soluble base. Enamel paints are made from an oil base. For exterior house paints, both enamel and latex paints are commonly used.