How can you tell if a reaction will happen?

This will depend on the conservation laws. In the same way that a numerical equation must be equal on either side, a reaction must have the same amount of certain properties on each side. The properties that are always conserved are baryon number, charge and lepton number. Lepton number is split into 3 types, electron, muon and tau, and so the number of each type must be conserved, and not necessarily the total number of leptons.

The last property is strangeness, which is always conserved except when the particles involved interact via the weak interaction. The weak interaction is carried by the W and Z gauge bosons, and can be identified if the reaction involves neutrinos. The strong interaction will involve only baryons, and the electromagnetic interaction will ususally involve photons; this information can be used to verify strangeness as a conserved property.