You can see if they are mature on the plant like LegalizeDaHurb says, they stick out with the familiar brown, and you can pinch them to check if they are hard. It takes 2-3 weeks for the seeds to mature. they fall right out when the bud is dried (a little help of course) I had two plants last grow go hermie very late in life, they produced some 60 seeds (between the two) and at least 20 from each plant are mature enough to germinate. Very glad my sativa dom seeded.