In order to identify transitional flow, the linear stability of inward flow in a narrow gap between co-rotating disks is studied. Two methods are developed. The local approach assumes periodic nature of the flow in radial and circumferential direction, whereas the computationally more expensive biglobal variant only requires periodicity in circumferential direction. Stability maps are provided for a single set of operating conditions spanning a range of Reynolds numbers at fixed rotor geometry and rotational speed. The theoretical findings are compared to a set of experimental velocity profiles that includes the transitional region according to profile shapes. Apart from some explainable inconsistencies, the determined stability limits agree with each other and with results from previous studies.