The inventive subject matter relates to a method for detecting the presence of a biological substance of interest in a test sample of saliva or oral fluid, comprising combining said test sample with a fluorescence-labeled ligand to said biological substance and detecting a change in the fluorescence polarization of said test sample produced by binding of said fluorescence-labeled ligand to said biological substance. In one aspect of the inventive subject matter, said method comprises additional steps for comparing the fluorescence polarization of said test sample with the fluorescence polarization of a control solution. Also provided is a miniaturized, portable apparatus for measuring the fluorescence polarization of a liquid sample.