Built-in-Self-Test (BiST) for transmitters is a desirable choice since it eliminates the reliance on expensive instrumentation to do RF signal analysis. Existing on-chip resources, such as power or envelope detectors or small additional circuitry can be used for BiST purposes. However, due to limited bandwidth, measurement of complex specifications, such as IQ imbalance is challenging. Since these parameters are most amenable for digital compensation, their characterization and monitoring are desirable. In this paper, we propose a BiST technique for transmitter IQ imbalance using a self-mixing envelope detector. We first derive an analytical expression for the output signal. Using this expression, we devise test signals to isolate the effects of gain and phase imbalance, DC offsets, and time skews from other parameters of the system. Once isolated, these parameters are calculated easily with a few mathematical operations. Simulations and hardware measurements show that the technique can provide accurate characterization of IQ imbalances.