I don't know the rifle but for PA longrifles some key clues are buttstock width, how flat the buttplate is, dating the lock, style of the furniture, and carving and patchbox styles. For a Revolutionary War period rifle look for a buttplate close to, if not over, 2" wide, with little curvature, a grip rail well off the stock, a wooden patchbox, or an early style brass box, etc. Later rifles will have narrower buttplates with more curvature, a guard with a narrower bow often closer to the wrist, a later lock, and later styled patchbox.
Those so just rough basics. Thickness of the barrel at the breech, his much it is swamped, and other factors are taken into account. When a maker has left behind an impressive body of work and his working life is known, it is easier to place particular guns on a rough timeline.