Titrate a known mass of diphenylacetic acid in anhydrous tetrahydrofuran (THF) against your organolithium reagent until a persistent pale yellow colour appears. RLi deprotonates the carboxylic acid to form the lithium carboxylate (colourless), with any RLi added beyond that point deprotonating the weakly acidic benzylic C–H, generating a coloured carbanion.
To a round-bottom flask was added 200 mg of diphenylacetic acid and THF (2.00 mL), under an argon atmosphere. Then, the organolithium reagent was added dropwise until the expected colour change was observed.