I always recommend fitting garments on a real person and not just a dress form. Dress forms are great fitting tools, but there are some benefits of fitting on a person that a dress form can’t replace.
For me, the benefits of fitting on a real person are:
She can tell you how easy/hard the garment is to get on and off.
She can tell you how the seams on the inside feel against the skin (especially important when designing for sensitive skin or for people with sensory sensitivity).
You can see how the garment and fabric look in motion.
She can tell you how the fit and style makes her feel. You can often tell by a person’s expression whether the garment makes them feel more confident in themselves or not.