This paper develops a lumped parameter model-based method for membrane fouling detection in Direct Contact Membrane Distillation (DCMD) process. First, a previously published mathematical model of DCMD is extended to account for the thermal resistance of the fouling layer. Then, an adaptive nonlinear descriptor observer is developed. The adaptive observer estimates the thermal resistance of the fouling layer in addition to the temporal and spatial temperature distribution of the bulk feed and permeate solutions and at membrane interface layers. Simulation results are presented to illustrate the performance of the proposed method.