Due to the existence of the 2 possible final states, each β decay is a mixture of the two decay types. This essentially means that some of the time the remaining nucleus is in an excited state and other times the decay is directly to the ground state.
Unlike Fermi transitions, Gamow–Teller transitions occur via an operator that operates only if the initial nuclear wavefunction and final nuclear wavefunction are defined. 
The Isospin and Angular Momentum selection rules can be deduced from the operator and the identification of allowed and forbidden decays can be found.
;Examples:
or
The above reaction involves "mirror nuclei", nuclei in which the numbers of protons and neutrons are interchanged.
One can measure the angular distributions of β particles with respect to the axis of nuclear spin polarization to determine what the mixture is between the two decay types (Fermi and Gamow–Teller).
The mixture can be expressed as a ratio of matrix elements (Fermi's golden rule relates transitions to matrix elements)
The interesting observation is that y for mirror nuclei is on the order of the value of y for neutron decay while non-mirror nuclear decays tend to be an order of magnitude less.