You can tell if a rainbow shark is a boy or a girl by observing its body shape and coloring. Female rainbow sharks tend to be thicker and slightly bigger in size compared to their male counterparts.
Additionally, male rainbow sharks are typically brighter in color than females, and also have easily identifiable black lines on their tails and anal fins that females lack.
Note that determining a rainbow shark’s sex once it’s sexually mature is much easier than trying to do so when they are young. A rainbow shark typically reaches sexual maturity when it’s around one or two years of age.