How do you test paint on the wall to find out if it is latex or oil?

Wash a small area with a solution of houshold detergent and warm water. Rinse well and dry it, then soak a soft rag in alcohol and rub it back and forth over the area. If paint comes off, it's latex paint and you can use another coat of latex paint. If the paint doesn't come off, it's oil-based, an oil-based primer is a must