This can be seen in esr. The odd electrons coupled to the H's. There are several very real problems with resonance theory. Although one particular resonance structure can make a minor contribution, a large number of them can add up, obscuring what one would like to be a simple picture. The classic example is again benzene.
Experimental probes have long time periods so only averaged structures are often seen.
We'll talk much more about cyclobutadienes later this semester, but the weight of experimental evidence suggests that it has a rectangular shape. Often times x-ray structures, even when available, give ambiguous results because of disorder or because the effects of substituents mask the situation. Little direct structural information exists because the molecules are very unstable and in an crystalline environment the interaction between the carbonium ion and its counterion may well cause the structure to be distorted from its solution or gas phase structure, where this type of interaction probably does not exist. Let us first suppose that this is an equilibrium reaction, where C is a transition state. Provided that R R', B will give two distinct signals, too.