With the advent of disposed syringes with hypodermic needles on the urban road sides and suburban parks and trails, there is an emerging concern about using finger drag tests for discovering hard to see causes of flat tires [to digress, not all causes of flat tires are immediately evident].
Sounds kind of paranoid to me. What are the odds that you would get a puncture from a syringe, prick yourself, and contract something. Got to be one in a billion or something. I would be willing to bet that this is something that has never happened to anyone anywhere and never will.
That said I have seldom resorted to the finger drag test. If I know how the tube was in the tire and find the hole in the tube, I then have a very small area of the tire to look at. At that point I have always been able to find the offending object visually as easily as with the finger drag method. The times when it is hard to find it visually there is nothing sticking out so the finger drag doesn't work either. In those cases flexing the casing (sort of pushing a section inside out) in the suspect area sometimes helps.
I have heard that dragging a cotton ball will leave a bit of cotton on the object, but I have never tried it.