It's important to test paint type to know what the existing coating is that you are about to paint over.
This is crucial to knowing what type of paint you need to apply as your over coats.
The easiest and least expensive is to test with methylated spirits, what you need to do is rub the methylated spirits on the surface to be painted. If the surface coating dissolves or becomes soft the surface is an acrylic paint, and can be over coated with any acrylic coating.
If the surface doesn't change when you rub it with methylated spirits, it is likely to be an enamel paint coating, therefore it is recommended that you lightly sand the surface with a medium to fine sand paper (120grit will do the job well) then apply one coat of oil based undercoat, then you can over coat with either acrylic or oil based top coats.
We recommend that if you are painting doors, windows, frames or any surfaces that may rub you use oil based top coats because they are a lot harder wearing.