What simple experiment could you conduct to test the hypothesis that hatchling sea turtles use ...

Attaching a magnet to a turtle's shell prevents it from sensing Earth's magnetic field, whereas attaching a nonmagnetic piece of metal does not interfere with magnetic orientation. In an experiment using this difference, hatchling loggerhead turtles from the east coast of Florida oriented to the east when a brass bar was glued to their shell but showed no orientation when a magnet was glued to the shell. (W. P. Irwin and K. J. Lohmann. 2003 . Magnet-induced disorientation in hatchling loggerhead sea turtles. The Journal of Experimental Biology 206:497-501)
A more difficult approach is to alter the orientation of the magnetic field the turtle senses by passing an electric current through a wire coil surrounding the experimental apparatus. In an experiment of this sort, hatchling loggerhead sea turtles tested in Earth's normal magnetic field oriented in a northeast direction, which is the correct heading to carry them from the test site on the east coast of Florida to the Gulf Stream. When the magnetic field was reversed by 180 degrees, the turtles shifted their orientation to the direction that they perceived as being northeast but was actually southwest. (K. J. Lohmann. 1991 . Magnetic orientation by hatchling loggerhead sea turtles (Caretta caretta). The Journal of Experimental Biology 155:37- 49)