"Dennis" wrote
Most likely they used two layers of 15# felt. (actually it's required in
Ohio because for ice protection at the eaves.) The felt is double layered
and cemented together up to a point 24in. in from the exterior walls. If
the roof slope is 2:12 up to 4:12 (low-slope) a double layer is still
required for the rest of the roof (otherwise 15# will be code).
About the only way you can tell is to look under the shingles at the edge
of
the roof. You may be ably to see if it's one or two layers. (Look about
mid-roof).

I'm licensed in seven (7) cities in NorthEast Ohio, and never heard of two
layers of felt.

Ice protection at eaves, two feet into the interior space, is a requirement.
Two layers of felt would not be allowable to substitute for ASTM D 1970.
Two layers of felt does not meet standard ASTM D 1970.

Always use required materials, otherwise you will be red tagged. In this
case, it would be an ice/water guard.