We can infer specific properties of the virtual particles. Mass, spin, charge among others, depending on what we’re talking about. If those properties match up with expectation, we consider this strong support of the theoretical formalism.
We can even measure more fancy stuff like virtual loop contribution to transition processes. This mostly works by measuring the transition probabilities with extreme precision, and then comparing it with higher-order theoretical predictions of the probabilities.
We can even measure more fancy stuff like virtual loop contribution to transition processes. This mostly works by measuring the transition probabilities with extreme precision, and then comparing it with higher-order theoretical predictions of the probabilities.