I assume from your post that the heads are on the work bench. If so, and the coating of carbon is light, you can use good old brake clean and a rag. I do this on heads that are lightly carboned and it works great. Spray it on, leave it sit a minute or two and wipe the carbon off. I did this last August to an engine I put in my Camaro that had approx. 13,000 miles on it. I had the heads off and I cleaned all the carbon off the heads and the tops of the pistons. Good Luck.
ASE Master Engine Machinist and Assembly Specialist. Building engines for over 30 years.