How to determine if a given reaction requires heating to initiate?
The ease of initiation of a reaction depends on the activation energy required. In order for reactions to proceed (at a reasonable rate), the forces binding the individual reactants must be overcome, which may require energy, solvent, catalyst or other input.

For example, $\ce{H2O2}$ is quite stable at room temperature, but exposure to horseradish peroxidase, silver oxide or other catalyst causes rapid decomposition. Iron filings do not oxidize quickly, but apply a bit of salt water and they rust so quickly they make an effective heater. $\ce{NI3}$ has an extremely low activation requirement!