The diagnostics used to determine the axial potential profiles in the Tandem Mirror Experiment-Upgrade (TMX-U) are described, together with the initial results of these measurements. The application of electron cyclotron resonance heating (ECRH) in the anchors is found to create high central-cell potentials. When the end walls are grounded, deep thermal barriers are created by the application of second harmonic ECRH. Rapid decay of these barriers suggests that they are formed by the presence of low temperature, magnetically confined electrons. Axial confinement of the central-cell ions is sometimes poor, even in the presence of a deep thermal barrier. Frequently, a potential maximum is observed near the inner turning point of the anchor sloshing ions. This potential is sometimes large enough to explain the reduced axial current from the central cell.