How do I know if I need a prehung door or a slab to replace my door?

if the frame is sound and still 'in square' a single door will work -- if the frame is warped/damaged you'll need a prehung with the frame.

To put in a prehung door, you must shim the jam until it is level. And you will have to replace the trim. To put in a slab, you have to bore a hole for the doorknob that is exactly where the strike plate on the old jam is. Then you have to chisel out spots for the hinges that also match up to the existing jam.

If you keep your frame and buy a slab, you will have to router out where the hinges and hardware go and it can be pretty difficult to line up - definitely a job for a professional or someone with experience.

I would say for an ameteur, the extra $ you spend on a prehung door will be saved in time and frustration dealing with a slab.