Fluorescence techniques can provide insights into the environment of fluorescence indicators incorporated within a ligand as it is bound to its receptor. Fluorescence indicators of different sizes and chemical characteristics can provide insights into the nature of the binding environment, the surrounding structures, and even into conformational changes associated with receptor activation. Methods for determining fluorescence spectral analysis, fluorescence quenching, fluorescence anisotropy, fluorescence lifetimes, and red edge excitation shifts of the ligand probes are described. The applications of these techniques to the CCK1 receptor occupied by alexa488-CCK and aladan-CCK, as examples of probes developed (1) by derivatization of an existing peptide and (2) by incorporation during peptide synthesis, are utilized as examples. These methods represent powerful tools to expand our understanding of the structure and molecular basis of ligand activation of G protein-coupled receptors.