I have to paint the walls and trim in a house and they think the trim is painted with oil but not sure. Is there any way to test it?
Stick it with Acrylic
With the advances of acrylics in the last ten years and present day technology, a good sanding and proper prep, acrylic paints will stick to most surfaces better than oils. (ie. aluminum, masonry, oil based finishes) Oil based alkyd formulation has not changed in fifty years, but you still can't beet the look and feel of a good hand brushed oil finish.
It has been my experience that latex WILL NOT STICK to oil base paints. I have painted many historic homes and they have all had old oil base paints on the trim. I learned the hard way that latex will not stick to oil base.
As has been stated in this forum, preparation is the key. The denatured alcohol test is by far the most simple test. After the oil/latex determination, visit your local paint store to get their recommendation on the appropriate primer / finish system. Acrylics, with the proper prep, are a fine interior finish over previously painted surfaces.
We have begun to use the new waterbourne finishes with great results. Several of these products combine the durability of an Alkyd finish with the ease of clean-up and resistance of yellowing of the acrylic finishes.
I will caution that the waterbourne finishes are not your grandmother's coatings. Expect a learning curve the first few times these products are used.
I apply them on cabinets or trim and am getting great results. I must teach my painters the application process as these coatings can not be applied as liberally as most other interior coatings.
RUB DENATURED ALCOHOL ON THE AREA, IF THE PAINT IS SOFT OR COMES OFF IT'S A LATEX PAINT, IF IT STAYS THE SAME IT'S OIL BASED.