Part of our continuous efforts to implement an effective closed-loop feedback control of the flow over a NACA 4412 airfoil is the obtention of an accurate estimate of the actual flow state. An elaborate controller combines both prediction and measurement techniques to obtain a precise estimation of the control variable. In this paper we focus on the measurement process. We first present the different candidates for the control variable, and describe the low-dimensional techniques employed. We then focus on the estimation methods that will be incorporated in the controller in order to access the state variable from the pressure real-time measurements. The investigation of the dynamics in the flow field and the correlations between the variables at stake reveal the benefits brought by a spectral estimation approach.