The Belgian professor Joseph Plateau invented the stroboscope in 1832. Plateau created the first Phenakistoscope by cutting slits into a disc which he turned while viewing images on a separate rotating wheel. It was almost simultaneously invented by the Austrian Simon von Stampfer. Stampfer called his a "Stroboscope"the nomenclature we use today.
A stroboscope for phonographs is a paper disc placed at the center of the platter. On the disc is a circle of equally spaced lines. When viewed under an electric light the record is spinning at the correct speed (in this case 78 rpm) the lines on the stroboscope appear to be still.
What is happening is that the electric light is actually blinking on and off 3600 times a minute (3000 at 50Hz.) Each time the light goes on, the black lines on the stroboscope have rotated one space if the stroboscope is running at the proper speed. The book The Compleat Talking Machine by Eric L. Reiss has some simple instructions on making one or you can just print out the one on the W.A.M.S. webpage here.