Check out Revenuecollector's wonderful website. https://revenue-collector.com/pages/3552.html
A quick bath in some distilled water should loosen the fibers if they re not attached to the paper. You can use a small soft paint brush to wisk away any material the water may have loosened. Typically the blue drug wrapper fibers would be stuck to the back of the stamp as it would have been adhered to the wrapper by the adhesive. If the fibers remain, I would conclude that it is a silk variety. Note, that most often the blue, and sometimes red, fibers are more prominent on the reverse of stamp. In any case Revenuecollectors site is an amazing resource for collectors of such material.
Edit to clarify posted link. The value of the stamp linked is due to the double transfer, not being silk variety, it has a very small catalog value as silk.