Apparatus and methods of monitoring gastric restriction devices are described. Internally mounted sensors detect at least one of a quantity of a test substance, a flow through the stomal opening produced by a restriction device, slippage of the device, and erosion of the gastric wall. In some embodiments flow versus no flow can be determined, or a flow rate can be calculated. Monitoring of internally mounted sensors permits optimization of the performance of a gastric restriction device, using noninvasive techniques.