Internet of Things (IoT) nodes used in environmental monitoring and smart city applications are becoming increasingly prevalent with over $200B projected market potential. These nodes typically employ one-way communications using a high-end transmitter without a corresponding receiver. Testing of such transmitter-only systems poses an additional challenge. Due to the lack of a receiver, low-cost test techniques, such as loop-back, cannot be used. In this paper, we present a low overhead built-in self-test (BIST) technique to characterize imbalances of IQ transmitters without a receiver, both for post-production and in-field test purposes. The proposed BIST uses simple circuitry and a single test setup. The target parameters are analytically computed independent from internal BIST parameters which eliminates the need for initial calibration phase. All measurements are in DC and no external RF signal generation is required. The overall measurement time, including the computation time, is less than 2ms. Simulation and measurement results show that the proposed method provides adequate estimation accuracy for digital calibration.