Hersh -
There's no way you could spot a fractured spar (or similar) on a walkround
I don't know about that. I'll admit to the possibility that some forms of internal damage may leave no external sign, but there are quite a few ways such damage might reveal itself, albeit subtle. I have always made a practice of carrying out JF's advice about standing back and looking at the aeroplane from a reasonable distance, from different angles. Does the dihedral look right? From square on the nose, does the tailplane make a right angle with the fin? Do the tips of the tailplane tappear to touch the visual surface of the wings in the right place? Is there any unusual washout or washin? Closer in, perhaps using reflections off the skin, are there any signs of stress such as faint wrinkles running diagonally across metal or fabric panels? Are there any rivets or stiches which are starting to pull or come loose? If you push up carefully on the wing tip, does the wing take up the strain evenly, and are there any sounds you wouldn't expect? Do the doors and removable access panels fit in the same way they always have, or is there a bit of a struggle to get latches / fasteners home? Are hinges rubbing or binding? On biplanes of course you also have the condition and tensions of the rigging wires to look at.All these could be clues to something having changed inside the structure.