A general method is proposed to determine the ability of a vector-sensor antenna to estimate the electric and magnetic fields at one point. This approach works regardless of the positions and types of the antenna elements, and can take into account their mutual couplings. This method is based on a matrix, which computation involves reciprocity and two modal representations of the fields. Its condition number determines the capability of a configuration to estimate the direction of arrival in presence of spatial noise (multipath). Simulation results confirm the validity of this approach.