First, you need to determine the relative stability of the compounds. Usually, this can be known by comparing the preferred state of the element or compound to its current state. For example, metals commonly want to donate electron so seeing a negatively charged metal should give a hint that the molecule/atom you're dealing with is unstable.
Second, the spontaneity of the reaction is a big help in determining if the two molecules would atleast react (the spontaneity is given by how negative the change in gibbs energy is).
Third, compounds with variety of resonance structures prefer to be themselves because the electron density is delocalized in the molecule.
Lastly, reactivity of the reactants is accompanied by energy. Presence of heat, current, work, etc. usually pushes the equilibrium of the reaction to favor the formation of the product. However, not all reaction have this kind of characteristics especially exothermic ones.