Spatially resolved photoluminescence spectra of thin GaAs quantum wells are measured by near-field spectroscopy, and two-energy autocorrelation functions of the spectra are derived. We demonstrate distinctly different autocorrelation functions for a 3-nm-thick quantum well grown on a (311)A surface and for quantum wells grown on (100) substrates. The autocorrelation spectra of the (311)A GaAs quantum well are quantitatively described by a statistical disorder model with a single correlation length of 17 nm for the exciton center-of-mass motion. A shoulder in the autocorrelation spectrum at energies between 1 and 3 meV is identified as a signature of excitonic level repulsion in the disorder potential. In contrast, the characteristic feature in the autocorrelation spectra of the (100) quantum wells is an additional positive correlation peak at energies between 3 and 4 meV. This peak reflects the energy correlations between ground and excited exciton states in individual monolayer islands with a narrow size distribution. The results indicate that the disorder potential in these high-quality (100) GaAs quantum wells contains both contributions from monolayer islands and nanoroughness on a length scale of the exciton Bohr radius.