You can demonstrate ET effectively by using a very very tiny quantity of one (the most active developer) and a very large amount of the less active or inactive developing agent.
One is quite active by itself, but in too low of a quantity to do more than just barely work and the other does virtually nothing by itself.
This argument is only true at the chosen pH.
That is one method of testing.
It is hard to differentiate between synergy and ET development. In synergy both developers are used at a relatively similar concentration and could do the job alone, so for example many developers with HQ and Metol can be reformulated to use just Metol at the right concentration. And they use each at about the same amount.