In this paper, we propose a generic method to assess the vulnerability to Differential Fault Analysis of generalized Feistel networks (GFN). This method is based on an in-depth analysis of the GFN properties. First the diffusion of faults is studied, both at the block level and at the S-box level, in order to have a fault which maximizes the number of S-boxes impacted by a fault. Then the number of faults in an S-box required to find the key is evaluated. By combining these results, a precise assessment of the vulnerability to fault attacks of GFN can be made. This method is then used on several examples of Feistel ciphers.