Based on the small sample I have, 2 Shell Lakes and 3-4 T's,
if the planking looks nice, ie, stright tight grain, layed tight, etc, it's a T, if the grain of the planking is more "open" and there are "gaps" in the planking, it's a SL.
Another clue, if the ends of the ribs have a small chamfer for and aft, it;'s likely a T, if not, a SL.
Oh, another thing I noticed on mine, the T's have a nice smooth shape and the SL's kind a look like a "sausage", ie, they stay wide in the middle for a long way and suddenly and sharply curve in to the ends.
Dan