Measuring how much a head was shaved?

Ok....so I'm home recovering from surgery and I am finally able to limp out to the shop this morning.

So I'm cleaning and organizing my engine assembly/parts area and I keep staring at my shiny redone W-2's and I wonder "how much have these heads been milled"?

They were new in '94...ran 18 nights...sat under the bench until 2007 then it was loaned out by the PO and the motor blew.

I bought it broken three years ago.

So my shop redid the heads, new valves, etc and took a skim cut. The oring grooves are still visible from the original build.

Where do I measure?