Hi Paul. There are a few different ways you can determine the gender of cannabis plants.

With female plants, the flowers grow tiny hairs around 4 – 8 weeks past germination depending on the strain. These hairs, or what are called Pistils are a frosty white cluster of little shoots that look like fine hair.

Regarding a male plant, instead of the flowers, male cannabis plants grow what are called pods around the 3rd week of growth from a seedling. These pods appear as clusters around the protruding stalk, often confused with the female’s Calyx. This is where the seeds begin to form.

To learn more about cannabis gender, read these articles:
How to Identify Male vs Female Marijuana Plants
How to Determine Early Signs of Hermie Plants