Measure the volume of your gear plus the amount of food (by bulk) and water you usually carry. Use a box with straight sides and be sure the top surface of your gear is level. Measure length, width, height; multiply them by each other. Assuming you measured in inches, 60 cubic inches equals approximately one liter (close enough for estimating purposes).
As the SOTM articles have shown, pack capacity is not always accurately measured by the manufacturers. Have all your gear (including the equivalent of food and water) ready to load the pack as soon as it arrives. Also do a "hike" around the house for several hours, to be sure the pack is comfortable for you as well as holding your gear.