I got the bike back together and after cleaning/polishing I always wipe the rotors with some brake cleaner.
I shoot a little on a clean paper towel and then hold it against the rotor and spin it a few times. First time the towel usually has a darkish spot and the second time the towel is usually clean and I'm done. The cleaner is supposed to be pretty strong and leaves no residue it says.
I don't know why but after the second wipe I put some spit on the towel and spun the wheel again. This time it came up dirtier(or at least blacker)than the first wipe with brake cleaner. Not sure why spit would remove something the cleaner doesn't but now I'll be spittin' on the rotors before I consider it done. Yeah, water will do.