My guess, is that Voyager being declared "outside" the solar system will happen only after quite of bit of time has passed since it actually left. Like the Higgs Boson, there is data, there is analysis, and there are confidence intervals. One result may be that the instruments send back cosmic ray information that no theory can explain how that would be consistent with being inside the heliosphere. Or it may simply be that the environment stops changing. And after a while that non-changing result is accepted as being "outside" and then you can walk backwards in the data to figure out the first time you got that result (and consequently the last change you saw) and declare the exit that way.