Retinoid-binding proteins (RBPs) are a diverse category of proteins that have been most extensively characterized for their role in vertebrate development. Recent work has uncovered new functions of RBPs in invertebrates and plants. Here, we present a methodology for applying a fluorescent chemical probe to characterize RBP binding in plants. This reporter, called merocyanine aldehyde (MCA), fluoresces upon binding to RBPs and therefore enables in vivo investigations into their functions with high spatio-temporal resolution. MCA treatment is simple, fast, non-destructive, and does not require prior knowledge of the RBP encoding genes. Therefore, a major advantage of this methodology is that it can be performed in species that are not genetically tractable. Furthermore, many of the methods presented here apply to diverse species within and beyond the plant kingdom.