how to locate spots to drill
 

Got an old 18' CC Proline. I want to put some rod holders on/in the
gunwales for outriggers and trolling.

I know the areas where I'd like to have the rod holders (basically it's
where I can simply lean right or left at the wheel and tend the rods. And,
then a pair up just forward of the console so the guy in front can use a
couple rods).

There's good flat area along the spots where I'd like the holes to go.

The big question is how the heck do the guys in the boat install/repair
business know what's under a given spot, and whether it's OK to punch down
through that particular spot?? How can I maximize my chances of the spots
where I want to put the rod holders being OK for that purpose?