What physical tests are involved with verifying a wing spar is suitably constructed for a homebuilt aircraft, such as the piper cub kit from wicks?
For a homebuilt a static load test is required. Whereas a full certification would require a loading to 1.5 the maximum load, this factor is reduced for experimental aircraft to 1.0. But performed it must be nonetheless.

For the test the aircraft is propped up upside down and sandbags (or water bottles) are placed on the wing until the calculated load is reached.



Structural test of the Electra One (picture source)