Vortex breakdown is currently identified in numerical flow solutions through postprocessing techniques. Although these methods can identify breakdown accurately, they can involve cumbersome data transfer and processing that can be quite time consuming. Therefore, a simple Rossby number criterion is proposed. The criterion is established using solutions to Euler and Navier-Stockes equations for low-speed flow over a flat-plate delta wing at high angles of attack. Comparison with experimental data shows consistent values of Rossby number at break-down. The proposed Rossby number criterion provides an efficient means of locating breakdown in numerical solutions. It is also straightforward to incorporate in any flow solver