An explosion in a custard factory
but also some of the chemical constituents of custard powder


Don't blame it on the custard. As long as there's 


dust of a combustible material, preferably with a small particle size
sufficient oxygen in the athmosphere
an igniter, such as a spark or an open flame


there's a good chance for a devastating dust explosion.

Your combustible material can be something seemingly unspectecular, coal dust, saw dust, flour, or custard powder. 


UPDATE

The incident in the BBC game most likely took place at a factory in Banbury, UK in 1981. Their traditional custard powder, known as Bird's Custard, is egg-free and mostly consists of cornflour. 


  how you might use it to tell that the reaction was an explosion in a custard factory


In risk assessment for dust explosions of organic materials, product analysis rarely plays a role; the "equation" isn't interesting and it is usually assumed that all carbon ends up as $\ce{CO2}$.

In order to establish workplace safety, it is more important to determine, how "strong" a possible dust explosion might get.

A measure for the "strongness" of an explosion is the rate by which the pressure (rapidly) increases. This is expressed in the $\mathrm{K_{St}}$ value (in $\mathrm{bar\cdot m \cdot s^{-1}}$), and experimentally determined in standardized tests.

Based on the $\mathrm{K_{St}}$ value, materials of certain properties (particle size, rest humidity) are assigned to the explosion classes (ST 0 to ST 3), and suitable workplace safety measures are taken.