It is not an assumption. Once a plant is mature enough it will show you it's sex no matter if it's in veg or flower. When your plant throws you pistils in veg, there are not assumptions to be made, a pistil is a pistil.It usually needs to be on a 12/12 light cycle to be able to tell the sex for sure. Some say you can tell before that but I would say it's kind of risky to assume..Good luck!