We consider the multiple candidate OFDM systems as a class of probabilistic method to reduce the peak to average power ratio (PAPR) of OFDM symbols. The candidate having the lowest PAPR is selected for transmission. For that, many IFFTs have to be processed to measure the PAPR of each candidate, which can be too complex for handheld devices. We propose the concept of phase uncertainty as an alternative measurement to PAPR for PSK modulations.We show the correlation between the phase uncertainty (PU) and the maximum amplitude. We propose a suboptimal (low complexity) algorithm to estimate this PU. Simulation results illustrate the performance of this algorithm.