Semiconductor power devices have a growing popularity in industrial, scientific and medical (ISM) band applications, such as microwave-heating, drying and plasma generation. A distinguishable property of these applications is the relentless pursuit of reliability, ruggedness, or the ability to withstand a mismatched loading condition without failure. Mismatch detection right at device output, preferably integrated in package, could provide critical information to avoid device breakdown and to enable performance optimization. In this work, the in-package mismatch detection and load monitoring is enabled by a specially designed six-port reflectometer. Compared with conventional reflection measurement techniques based on heterodyne receivers, the proposed design is passive, simple, compact and inexpensive. A hardware prototype is implemented with a size of 36 mm2. Loading conditions over the entire Smith chart are generated by a calibrated active-load-pull system and measured by the proposed system, yielding a root-mean-square error below -32 dB full-scale. This compact reflectometer enables the in-package accurate mismatch detection for the first time. The result of this study has been accepted for publication in the 2014 International Microwave Symposium, and nominated to be a finalist for its annual best paper award.