N_Cook wrote:
Is there a special probe for doing this, thin , tough, insulated , with graduations for indicating which row/column depth from the edge ?


I do not know, but it comes to mind you could use enamelled wire with the insulation removed on a small area at one end. Drawing a small scale on the wire may be the hardest part but if you are interested in just a few pins that reduces the number of marks to place.