This is more a question about the Higgs field than anything else. If you were to take, for example, a neutrino and send it out into empty space how could you determine that it has a mass in the first place unless it interacts with a particle? And on that level also couldn't one say that it's not possible to determine what we consider to be mass without the evidence of some interaction between particles?
closed as unclear what you're asking by knzhou, Jon Custer, ZeroTheHero, Chair, Cosmas Zachos Feb 7 at 2:13
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The rest mass of most fundamental particles (possibly not neutrinos) is a property of the quantum field. Specifically it is the Yukawa coupling to the Higgs field. Since the particles described by a quantum field differ only in their momentum, all similar particles have the same rest mass.
So we can be sure that an isolated particle in a vacuum has the same rest mass as one that is in some situation where it is interacting strongly.
Your choice of a neutrino as an example for your question is an awkward one since we don't know how neutrinos get their mass. However there is no reason to suppose that all neutrinos are not equal just as is the case for the other fundamental particles.