any means of test that leads to objective falsification of the hypothesis is valid. This includes, but is not limited to "experiments," "OBSERVATIONS," "counter-examples,"......
As I quoted previously, Einstein's General Theory of Relativity was "tested" not by experiment, but by OBSERVATION. Starlight passing near the Sun during a Solar eclipse was measured and compared to to the position of the same stars when they were NOT in the optical vicinity of the Sun, to determine if the light were bent, and by how much. The measured result was PRECISELY what Einstein's Theory predicted, but was NOT predicted by Newton's Theory of Gravitation. Thus, OBSERVATION was used as the method of testing Newton's and Einstein's competing theories of Gravitation. Newton's was thus falsified, and Einstein's wasn't.