Material extrusion (ME) additive manufacturing is known for its ease-of-use and high accessibility for users, yet there are limitations restricting its adoption. These limitations include high file-to-part and part-to-part variability and anisotropic mechanical properties. The objective of this research is to improve process visibility for the sake of modeling and control, to minimize defects and improve part consistency. An instrumented hot end was designed and manufactured to enable pressure sensing via a port perpendicular to the melt stream. The sensor port is in a lofted rectangular section of the melt channel to improve the signal to noise ratio. The pressure is measured via a load pin, which transmits the force to a 5 kg load cell adjacent to the hot end. Collection of pressure data at various flow rates and temperatures allows for characterization of the melt viscosity. This test cell also allows for characterization of transient effects, such as material compressibility. This work focuses specifically on characterizing poly(carbonate) (PC) and poly(lactic acid) (PLA) filaments with the aforementioned instrumented hot end. The apparent viscosity for each of these filaments is compared with its own respective Cross-WLF model, determined via offline capillary rheology. Compressibility effects are studied by running a Gcode designed to print a part with varying accelerations along its length and analyzing the pressure data collected during printing and the measured dimensions of the resulting part. The major takeaway from this work is that instrumented hot ends allow for effective observation of the melt behavior within the material extrusion hot end, providing a method for viscosity estimation and detection of transient effects present in the process.