How can you know if a particular piece of wing will generate lift? - - - Updated - - - If you're willing to install mods, I greatly recommend Kerbal Joint Reinforcement. It eliminates the need to strut wings even on larger designs. I have a Mk3 SSTO weighing 110t and able to lift about 45t into LKO. Naturally, its wings are huge, and without a single strut they hold together perfectly. This mod also greatly reduces the need for struts in all yur designs, including rockets. Give it a try.