Histological information is a good and trusted way to evaluate gonadal development. Hormone analysis can provide confirmatory evidence to your histological observations.

Its important to add that its ideal to do a thorough physical description of the gonad before proceeding to microscopic details of the gonads.

If you have any experience, you can try the ultrasound

Histology study is the best way rather then any other method. My suggestion is when sectioning the gonad we could easily identify what stage is based on its development of egg.

Studying vitellogenin levels can also aid such a study.