if you're building a room from the studs out this is a great article about remembering all of the different layers of construction and taking them into account when taking all of your measurements. all DIYers should pin for reference.
I did a bunch of googling and the consensus for testing oil vs. latex is this: Apply a small amount of denatured alcohol to a cotton ball. Rub the coton ball onto the painted surface. If the surface is latex, the paint will rub off onto the cotton ball. If it is oil-based, no paint will come off. Touch the area. If it feels tacky, the paint is latex and not oil-based. (Source)