How do we find the breaking point of an old braced wing?
Not sure what you mean by &quot;old braced wing&quot;, the Spitfires were full cantilever wings from the first prototype 300. From this link:

This link references the original Spitfire specification (without any further references to them):

5 Structural Strength
(a) The strength of the main structure when carrying the load specified in paragraph 3, plus 100 lb shall not be less than as specified hereunder ——
Load factor throughout the structure with the centre of pressure in its most forward position 9.0.
Load factor for wing structure with the centre of pressure in its most backward position in horizontal flight 6.0
Load factor in terminal nose dive 1.75
Inverted Flight
(1) Load factor at incidence corresponding to the inverted stall and with C.P. at 1/3 of chord 4.5
(2) Load factor at incidence appropriate to steady horizontal inverted flight and at the maximum speed of horizontal normal flight 4.5

Most WWII planes still flying are not stretched to the limit regarding g's, to keep exceed the fatigue life. I doubt any current owner would like to test what load the wings are up to at present.