In dual form, you're asking for conditions that characterize (or at least guarantee) that a given $L^1$ function is the Fourier transform of another function. To quote the introduction of Stein and Weiss' book on Fourier Analysis:
"Theorem 1.2 gives a necessary condition for a function to be a Fourier transform. Belonging to the class $C_0$, however, is far from being sufficient. There seems to be no simply satisfactory condition characterizing Fourier transforms of functions on $L^1(R^n)$$L^1(\Bbb R^n)$."