ok, it may not be marked but you have the side seam and it’s directly across from the Center Front (or Center Back). It usually has a slight curve at the top for the hip curve. You can do this same way that I do mine. Is your waistband just a rectangle shape? If you have a notch that matches up with an area of the pants, like the center front, you can measure to get the same distance to the original side seam, draw a line here, cut along this line and then spread apart the same amount that you’re increasing your waist by. This should mean that the notches will still match up will the new alteration. Don’t forget to make a muslin to test your pattern. Hope this helps.