Hydrogels are increasingly employed as multidimensional cell culture platforms often with a necessity that they respond to or control the cellular environment. Specifically, synthetic hydrogels, such as poly(ethylene glycol) (PEG)-based gels, are frequently utilized for probing the microenvironment's influence on cell function, as the gel properties can be precisely controlled in space and time. Synthetically tunable parameters, such as monomer structure and concentration, facilitate initial gel property control, while incorporation of responsive degradable units enables cell- and/or user-directed degradation. Such responsive gel systems are complex with dynamic changes occurring over multiple time-scales, and cells encapsulated in these synthetic hydrogels often experience and dictate local property changes profoundly different from those in the bulk material. Consequently, advances in bulk and local measurement techniques are needed to monitor property evolution quantatively and understand its effect on cell function. Here, recent progress in cell-responsive PEG hydrogel synthesis and mechanical property characterization is reviewed.