The Critical Mass Theory requires the aggregation of sub-particles to for particles. A simple experiment could involve determining whether concentrating sub-particles produces more particles. Implementing this experiment would involve setting up two experiments and measuring how many particles are produced.
For this example we use photons. In both cases there are two sources of photons.
Experiment 1: The two sources are directed at the panel that records photons such that they do not interact (i.e., the target are different positions on the panel with enough distance between the targets that they do not interact)
Experiment 2: The two sources are directed at the panel that records photons such that they interact (i.e., the target is the same position on the panel)
Expected result: Experiment 2 should create more photons than Experiment 2.
The complication of the experiment is that the concentration of sub-particles in experiment 2 must be enough that particles are formed. If this only occurs at the source (i.e., the concentration is only high enough exactly at the sources and the concentration diffuses rapidly), then the experiment will fail. It must occur where the sub-particles generated from the two sources overlap.