Debris flows can travel great distances from their sources and pose hazards to communities downstream. Identification of soils susceptible to liquefaction during slope failure could aid in locating potential debris-flow sources. We used a modified version of the slump test, typically used to measure the consistency of wet concrete mix, to test the proclivity of soil to flow. The method can be used to identify a threshold density below which a soil will flow under field-saturated conditions. We performed 51 soil mobility tests on 13 sandy and gravelly soils with 0%–16% silt and clay (fines) and one sample with 72% fines collected from landslide and nonlandslide sites. Results are consistent with corresponding site porosity and failure mode for coarse-grained soils. Threshold porosity for flow ranged from 0.39 to 0.65. Highest failure velocities were associated with the loosest soils and soils with the lowest permeability. Failure velocities and modes observed in our tests were consistent with test results for identical soils in a large-scale flume on thicker samples (0.65 m) that approach average failure depths measured in the field (0.75 m). This novel test was successfully designed as a rapidly conducted (∼24 h to prepare and test samples, including overnight saturation), low-cost alternative to large-diameter triaxial tests capable of testing gravelly soils that does not require a predefined shear rate, allowing pore pressure to evolve as soil fails under self-weight and drained conditions.