I am curious how this pattern was developed. Usually a grain line is the first element on paper when beginning to draft a pattern. Ideally, patternmaker's dot paper is used, this being like a 1" grid, so the horizontal hip line and center front are all there from the start. Drafting that grain line down perpendicular from the hip line will work to locate a grainline.
A fitting in muslin will help to show if you made the right choice, since a true vertical line will become apparent once the muslin is on the figure. Sometimes the body will 'torque' the fabric, so that true vertical and the grain line as drafted on the pattern do not align. This is where drawing a new grain line in colored pencil on the fitting muslin can help to re-locate that on the pattern. Tie a button to a length of thread and hang it down the front of the leg to locate the true grainline or vertical. This plumb line will also work to find the true alignment of the side seam.