Isn't it difficult to tell guenon individuals apart?
Not as difficult as you think. Though I admit that telling adult baboons apart during my time as a research assistant in South Africa was easier, adult redtail monkeys also show substantial inter-individual variation.
Though we primarily use features of the tail (Is the tail bent? Is it pointed? Is it more orange than red? Is if pure red or red and black?) to tell females apart, facial differences can be striking. Below are some examples. These images also serve as a facial recognition test that several people I know have already failed, so don’t feel too bad if these females don’t look that different to you -- although if you can’t differentiate Miss Piggy from the others, then you must accept that you have weak facial recognition abilities. That said, if you spend multiple field seasons with these redtail monkey ladies, the facial differences would become amplified for you too I’m sure.
Note: we use more practical feature-based code names for these females when collecting data, but they’ve also acquired the nicknames listed below.