For the future Smart Grid, devices called Phasor Measurement Units (PMUs) are being widely deployed to continuously monitor the state of the power grid in real-time. The voltage and current phasor measurements are synchronized across the network using GPS. However, because civilian GPS signals are unencrypted, these receivers are susceptible to being spoofed. In this work, we propose a wide-area spoofing detection algorithm for PMUs using a hybrid communication architecture which overlays NAPSInet, a proposed communication structure for the future Smart Grid. We create conditioned signal snippets which contain the military P(Y) signal, whose precise code sequence is unavailable to civilian users and thus cannot be generated by an attacker. Thus, the encrypted signal establishes a type of signature in the background of all authentic GPS signals, the presence of which can be verified by correlating between conditioned signals from other distant receivers. We further consider the potential for a coordinated spoofing attack against regional collections of cross-check receivers and generate representative snippets for each regional network to verify against other distant sites. Additionally, we use real-world data recorded during a government-sponsored, live-sky spoofing event and demonstrate our algorithm can successfully evaluate the authenticity of a widely dispersed network of receivers.