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).

" My mother had her house re-foofed and as I was cleaning up the yard I
found a felt paper wrapper for the 15 lbs kind. I had read the contract
and it called for 30, so I was really concerned my Mom had been ripped
off. I looked it up on the internet and confirmed it was definitely #15
(Esgard No.15 Asphalt Felt Plain).