Ice streams are bounded by abrupt transitions in speed called shear margins. Some shear margins are fixed by subglacial topography, but others are thought to be self‐organizing, evolving by thermal feedbacks to ice viscosity and basal drag which govern the stress balance of ice sheets. Resistive stresses (and properties governing shear‐margin formation) manifest non‐uniquely at the surface, motivating the use of subsurface observations to constrain ice sheet models. In this study, we use ice‐penetrating radar data to evaluate three 3D thermomechanical models of the Northeast Greenland Ice Stream (NEGIS), focusing on model reproductions of ice temperature (a primary control on viscosity) and subsurface velocity. Data/model agreement indicates elevated temperatures in the NEGIS margins, with depth‐averaged temperatures between 2°C and 6°C warmer in the southeast margin compared to ice in streaming flow, driven by vertical heat transport rather than shear heating. This work highlights complexity in ice divergence across stagnant/streaming transitions.