Latex or alkyd paint?

How can I tell if an old coat of paint is latex or alkyd?

Dip a rag in rubbing or wood alcohol and lightly rub the surface. If the surface doesn’t change and the rag is not stained, the surface is painted with alkyd paint. On the other hand, if the surface is altered and your rag is stained, the surface is painted with latex paint.

If the surface is white, use a dark coloured rag. If the surface has a colour, use a white rag.