I don't know about sage seeds but when chive seeds are ready the seed pods turn white an dry. Pick a few and rub them with your fingers/hand. They should come off easily and be hard black. ((if you wait too long they will fall on the groung and may grow next season)). Then you can cut all the seed bearing stems and set them on a piece of papaer (inside) untill they are fully dry. Then by rubbing the seed pods all the seed should separate.