No easy way if the plans don't include a bill of materials. Assuming you don't have the equipment and inclination to rip your own sticks and sheets from large balsa lumber, you'll need to measure and record each component on the plans, along with its dimensions and type of material (i.e., Spruce, 12"L, 14"W x1/4"H). When you've got a complete list, you can group by type of wood, and WxH, and start reducing it down where you can to standard lengths (i.e., 36" long for sticks, 3" or 4" wide for balsa sheet stock, 12" x12" or 12 x 24" for ply sheet stock, etc). For example, if you have 2 Spruce at 12"L and 1/4W" x1/4H", then you need to buy 1 36" piece. For wing ribs, you'll have to estimate how many you can fit on a single 3" or 4" x 36" sheet of balsa of the correct thickness.

You should probably plan on about 10% more of each type of wood than you calculate to account for bad pieces, mis-cuts, warpage, etc.