dial indicator will be more accurate. it wont tell you any thicknesses but it will show you if and where theres high and low spots and how much.
but like dc2GS-R said... take them and get them machined if possible. sometimes they cant be cut if they have worn too thin because you want them to be as thick as possible to disapate the heat better.
and on another note... you should machine your rotors every time you change your pads. youre actually supposed to. it will help prevent further damage to the rotor and you will be able to stop much better
