Soft-switching power converters based on wide-band-gap (WBG) transistors offer superior efficiency and power density advantages. However, at high frequencies, loss behavior varies significantly between different WBG technologies. This includes losses related to conduction and dynamic ON-resistance (R $_{DS(ON)}$ ) degradation, also charging/discharging of input capacitance (C $_ISS$ ) and output capacitance (C $OSS$ ). As datasheets lack such important information, we present measurement techniques and evaluation methods for soft-switching losses in WBG transistors which enable a detailed loss-breakdown analysis. We estimate the gate loss under soft-switching conditions using a simple small-signal measurement. Next, we use Sawyer-Tower (ST) and Nonlinear Resonance (NR) methods to measure large-signal C $OSS$ energy losses up to 40 MHz. Finally, we investigate the dependence of dynamic R ${DS(ON)}$ degradation on OFF-state voltage using pulsed-IV measurements. We demonstrate an insightful comparison of soft-switching losses for various normally-OFF Gallium-Nitride (GaN) and Silicon-Carbide (SiC) devices. A p-GaN-gated device exhibits the most severe R ${DS(ON)}$ degradation and the lowest gate loss. Cascode arrangement increases threshold voltage for GaN devices and reduces gate losses in SiC transistors; however, it leads to higher C $_OSS$ losses. The study facilitates the evaluation of system losses and selection of efficient WBG devices based on the trade-offs between various sources of losses at high frequencies.