Studying vertical sorting of particles is important for understanding the complex mechanism of debris flow. In this paper we proposed two methods to judge vertical sorting of particles in debris flow. One is based on the standard deviation of impact signals, and the other is based on the peaks in impact signals. We carried out three groups of flume tests with debris densities of 2 095 kg/m~3, 2 008 kg/m~3 and 1 960 kg/m~3, respectively. Results of the two methods are similar. There is no significant vertical sorting with the first group of debris flows. There is normal sorting with the other two groups, and the normal sorting becomes more significant for debris flows with smaller density. Dimensional analysis indicates that viscous stress is dominant in debris flow movement. It is difficult for the smaller stress between particles to make a reverse sorting. This is consistent with the results using impact signals. Therefore, the methods proposed in this paper are applicable to judge vertical sorting of particles in debris flow.