Oil paints tend to be very smooth and sleek to the touch, and latex paints have a more rubbery feel. The difference is more distinct with gloss paints than with flat paints. Beyond that, the easiest way to tell is to rub an area of the paint with denatured alcohol or a paint de-glosser. If the paint is latex-based, some of the paint will likely rub away and the surface will become slightly tacky. There will be no effect if the paint is oil-based.