Or voltage tester and measure the voltage from the white (neutral) wire in the 2-way box to ground (bare copper wire). It should measure 0 volts with the 2-way switch both off and on.
If it mesures 120 or so volts with it on, then it is being used to loop power back up to the light, and you cannot use it as a neutral.
Also a yes or no question..... Do You Have Two (2) Three Way Switches In The 3-Way Circuit Or Not??? Please answer Yes or No..