We propose a technique to determine one of three possible M-ary PSK modulations of a received signal without prior information. The three modulation types are BPSK(binary phase shift keying), QPSK(quaternary PSK), and 8-PSK(8-ary PSK). The detection of the modulation type is determined by observing the phase locked signal and signal points in the phase plane. A computer simulation indicates that the detection algorithm works proves useful.