How does one distinguish between the second-loop contribution of a known particle, and the first-loop contribution of a more massive-and as yet undiscovered-particle in the S-matrix and/or differential cross section?

Imagine: I calculate a one-loop correction to some scattering process in QED. My photon propagator needs to be corrected for all three lepton generations. I then calculate the differential cross section. The experimentalist then shows me a plot of his precise measurements and tells me that my calculations need further corrections.

Question: How do I know that the necessary correction is from two-loop diagrams, and not from a fourth lepton in the first-loop diagram that I haven't considered?

This post imported from StackExchange Physics at 2016-11-08 14:00 (UTC), posted by SE-user Optimus Prime