Histology of Ovarian tissues will be really helpful to ensure your results from L50.

You may use GSI and or histology to validate and support the results of your L50.

Good question...

As someone pointed out clearly, histology of gonads can be used to see whether fish is matured or not. But again, the question comes why you want to see the maturity in fish? Do you want to do any experiments or want to do some breeding trials? If your interest is to do the breeding, then try to notice the body colour and behavioural patterns. In general, most fish exhibits a change in body colour when they get matured; but again depends on the species...

Hope my answer is useful...Good luck!