The interference due to WiFi devices degrades the quality of the products obtained by C-band weather radars. In this work we present a strategy for detecting these interfering WiFi packets in the radar output signal. The method is based on a delay and correlate algorithm, which takes advantage of the periodic structure of the WiFi packets preamble, periodicity that is preserved even when the signal is distorted when passing through the radar reception stages. We formulate the detection strategy as an hypothesis test that uses the squared modulus of the auto-correlation as the statistic. With this formulation we obtain the decision threshold as a function of the desired false alarm probability. We perform a series of controlled experiments using real weather radar data collected by Argentinian C-band RMA radars. The results show a high detection rate both when the WiFi interference is in regions where there is only additive noise and when it is in regions where there is also a meteorological target.