Thanks for the input.
I still come back to the force it would apply to a wing (say, a wooden wing with fabric cover) if I was to try to lift the airplane at the tip. Surely the mechanical advantage I would have would be great enough to show up any weakness? In flight the force on the wing is spread out along the span and the force on the wing, especially if it has a strut in the mid span, would not be too great (considering maybe a 2 g load factor). But if I am trying to bend the wing from the tip, it would have a lot of force applied at the strut, the force I could apply multiplied by the distance from the tip to the strut??
When I have done this, I see the fabric loosen up on the top, showing that I am bending the wing a little. How much can I do this before I damage it? Or if it has internal damage, would this bring it to my attention? I guess I am not talking about DC8's , here, or any airplane that is substantially built, but it could apply to most ultra lights, sport airplanes and Cubs.
I fly many airplanes that are of suspect provenance, and would like a sure way to see if they are not at least going to come apart on me. I can see wrinkles and bad corrosion, but cannot see inside the structure.