Usually you can just go by the colors of the wood. Single ply maple shells, even with re-rings, will usually be the same light wood color all the way across.
When you get a mahogany ply tossed in it goes back and forth from light to dark. And even if you have a single ply mahogany shell with maple re-rings,
the entire shell portion should be that same dark mahogany color all the way across. A poplar or inner "soft wood" ply is almost always a lot thicker than the others.