How do physicists know that some of a beta ray/particle's 'missing' energy isn't lost to interference with the electron cloud surrounding the atom? Enrico Fermi and Wolfgang Pauli ultimately concluded that beta decay resulted in an electron and an electron antineutrino leaving a nucleus... BUT...
How does the electron leaving a neutron punch its way through the electron cloud surrounding a large nucleus?
How do we know that a slow-moving beta electron didn't just run through a lot of extra electrons surrounding the atom?...
 A: The electrons bound to the atom have quantized energy levels. A beta-decay electron can lose energy by causing an electronic excitation in the daughter atom, but only discrete energies can be removed.  If the beta electron were to excite the atom's electronic cloud with more than the binding energy of an inner electron, that inner electron would be ejected, and the atom would emit x-rays as the vacancy was filled.
A neutrinoless beta decay would have a monoenergetic energy spectrum, and a neutrinoless beta decay which lost energy to the electron cloud would be mostly monoenergetic but have features which correlate with the visible and x-ray spectrum of the atom.  For most beta-active nuclei, however, the energy released during beta decay is much more than the ionization energy of the atom.  There's just no way to generate an energy spectrum which varies smoothly over several mega-eV by sharing energy with an electron cloud in chunks of a hundred kilo-eV or less.
Furthermore, you can start with the angular momentum conservation laws for beta decays from different nuclear spin states, and from that you can figure out the distribution of angles between emitted beta electron and the emitted antineutrino.  Each angle is associated with a particular division of the energy between the beta electron and the antineutrino.  The predicted electron energy spectra are an excellent fit to the observed spectra.
Your question is the subject of several chapters in Allan Franklin's excellent book Are There Really Neutrinos; the history was complicated.
A: Beta particles have a typical kinetic energy of about half a million electron volts. This is plenty enough to pass through the electron cloud surrounding the nucleus and completely escape from it.
