How do I measure a small piston to head clearance

[Bluebell]

Hi guys I have a block with Ross 3 5/16" pistons. I have a pair of C7RA alluminium heads, which have been faced to get a good surface. If I put a head on loose with no gasket it rocks on the piston. If I put a used gasket on with the head and just hand nipped down on 4 studs, the engine turns without any noticeable contact. I'm guessing I have about )0.020" P. to H. clearance on this.
What is the best way to check a small clearance like this? and how do I remove the small amount of metal from the heads? Details are good.
I was hoping not have to remove metal from the heads.
Thanks in advance.
Red.