Flow separation over a two dimensional geometry such as aerofoil is defined as a point where shear stress goes to zero. For a three-dimensional boundary layer, separation is along a line and universally accepted definition of such lines do not exist for 3D geometry. Any flow leaves behind a wall signature on the surface which can be visualized by plotting velocity or skin-friction vector field over the surface. In our present work, critical point theory is used to explain the observations of vector fields over a finite wing surface and in particular, determine pattern in separation lines for 3D geometry. Oil flow experiments were carried out on a finite wing with NACA 0012 as cross-section for 0.085 and 0.1 million Reynolds number. Some of the results of wind tunnel experiments are discussed along with need for computational solution. Numerical simulations were carried out for sphere, spheroid, torus and 3D wing at different angles of attack at 1 million Re and separation lines are plotted over the surface. These lines are studied in pre-and post-stall angles of incidence to understand the nature of separation in three-dimensional boundary layer. © 2021, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.