Decay probabilities are rarely used, they depend on too many other things. Just the possibility of a decay channel is sufficient to classify the particle. Finding its actual spin value is then done by angular analysis of the decay products.
In case we suddenly forget all spin values:
Electrons have to be fermions, otherwise all electrons in atoms would occupy the lowest energy state and there would be no chemistry.
Protons and neutrons have to be fermions, otherwise the nuclide chart would look completely different (e. g. more stable large elements).
The decay of a fermion always leads to an odd number of fermions, the decay of a boson always leads to an even number.
In beta decays, neutrons decay to proton+electron+neutrino, therefore the neutrino has to be a fermion. It also follows that the W has to be a boson.
Electron+positron, both fermions, can annihilate to two and three photons, therefore the photon has to be a boson.
Quarks can radiate gluons, which looks like "quark -> quark+gluon", therefore the gluon has to be a boson. The same is true for Z, W, photons and Higgs.
Three valence quarks make up a proton or neutron, therefore quarks have to be fermions.
The Higgs can decay to two photons, therefore it has to be a boson.