After a few weeks, lightly tug on the stem. If the cutting resists, enough roots have formed for it to be removed and planted in a pot or outside.
Note: Many softwood cuttings can also be grown in a glass of water. With a sharp, clean knife, cut a 2- to 3-inch stem with several good leaves, and place it in a glass of water (make sure the leaves stay above the water line). Keep cuttings out of direct sunlight and make sure the water level stays constant. After a week or so, look for a developing root system. When the roots are about 1 inch long, it's time to take the cutting out and plant it in a soilless potting mix. Don't wait too long or the cutting will develop "water roots" that can absorb nutrients only from water and won't grow properly once they are planted in soil.