This protocol can be used to detect the amount and localization of endogenous phospho-Rab10 by light microscopy. Cells that yield detectable, endogenous phosphorylated Rab10 without the need to express LRRK2 include: Mouse embryonic fibroblasts (MEFs; wild type and LRRK2 R1441C or G2019S, VPS35 D620N); A549 PPM1H knock-out; NIH-3T3; immunopanned primary rat astrocytes. In our hands, HeLa, hTert-RPE, A549, HEK-293T, and ShSy5y cells can be immunostained for phosphorylated Rab10 but require exogenous expression of wildtype or pathogenic mutant LRRK2 or addition of pharmacological agents. Cells should be Mycoplasma free.