You don't. You assume that it is non-zero which is just a choice of "c".
The idea of Frobenius method is to write a solution in the form
[tex]\Sigma_{n=0}{\infnty}a_n x^{n+c}[/tex]
where c, while a constant, is not necessarily a positive integer.
If I, choosing c to be, say, c0[/sup], I find that a0= 0 but a1 is not, I could just take c= c0+ 1 instead.