The present disclosure relates to methods for determining a wavefront position of a liquid on a surface of an assay test strip placing a liquid on the surface of the test strip; and acquiring one or more signals from the surface of the test strip at one or more times, comparing the one or more acquired signals to a threshold, wherein the wavefront position is a position on the surface of the test strip where a signal is greater than or less than a threshold (e.g., fixed or dynamic threshold). Such methods may be used to determine the wavefront velocity of a liquid on a surface of an assay test strip and the transit time of a liquid sample to traverse the one or more positions on the surface of the assay test strip.