How can I get a list of the symbols in a sympy expression?

You can use:

f.free_symbols

which will return a set of all free symbols.

Example:

>>> import sympy
>>> x, y, z = sympy.symbols('x:z')
>>> f = sympy.exp(x + y) - sympy.sqrt(z)
>>> f.free_symbols
set([x, z, y])