My theory (as if anyone cares):
Testing of bio-load has to be through titration but instead of adding drops of some chemical, you have to keep on adding fishes till you see (whichever comes first):
1. Sudden growth of algae
2. Sudden deterioration of water quality
3. Do water change more than you would like to
When any of the three results matches, you have a heavy bio-load. Small bioload is the point where you don't see any fish in the tank (or you are an irrational environmental activist) and medium would be anything between small+1 and heavy-1.
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You wouldn't want to see my tank. I don't use fancy equipment and I am a noob 
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