With the advance of high power laser technologies, we are approaching the possibility to study strong-field vacuum breakdown and an accompanying electron-positron pair plasma via a quantum electrodynamic (QED) cascade. To reach a high quantum nonlinear factor in the particle rest frame, two different configurations are envisioned either through collision of a laser pulse with an energetic electron beam or collision of two laser pulses. Producing QED pair plasmas all optically is generally believed to require next generation laser technology that can output 100 PW power. Based on the electron-beam-laser collision setup, our recent work, however, shows that signatures of collective pair plasma effects appear in exquisite detail through plasma-induced frequency upshifts in the laser spectrum. Remarkably, these signatures can be detected even in a pair plasma created by passing a dense multi-Tens-GeV electron beam through a multi-PW laser pulse. This method substantially reduces the already low laser intensity requirement, and the use of lower laser intensities, compared to all-optical methods, significantly makes the QED collective effects easier to observe. Strong-field quantum and collective pair plasma effects can thus be explored with existing technology, providing vital information for QED cascades in general and QED plasma regimes in particular.