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.

I don't think you will see a sudden anything. I think of it as more gradual thing. It is a biological system, and, depending on your rock, sand, reactors, etc you may or may not be able to process a certain amount of fish waste. The bacteria will adapt and multiply to the point where they encounter a limitation of some sort (usually carbon). If you supply that, they will continue to grow to match the bio-load.

I also think of corals as consumers of nutrients, rather than adding to the bio-load. Am I wrong?

While they use nutrients they do not really remove them from a system just process them and get rid of what's not used, this may be food for other life including other coral . Certain things can not get processed out and eventually need some sort of export out of the system.

As for the size thing , its not always the biggest animals with largest bioload , snails , urchins and sea hares tend to do a lot of pooping lol

I would bet a sea hares waste is ten times as much as fish three times it's size.

So then, how do I estimate my bioload? Good question, huh?? :)

I always just get a skimmer twice rated my volume size , seems to work well lol

bioload
 

i have always gauged my bio load on type of fish and how many I have and size and feeding .I have a 90 gal tank and I think I am pushing the heavy side of bioload I don't think theres away to measure other then maybe skimate from your skimmer .I like the idea of a skimmer twice the size or close to more then half of water volume . just my two cents worth

Use a calculator..

http://aqadvisor.com/AqAdvisorMarine.php

Almost had it there :) Most fish missing from list.

Ya, works well. Guess there's no real answer to this :)

Way more complicated than I would have ever thought. After doing some reading the bioload also is affected buy amount of available oxygen, surface area (which is probably for oxygen) free swimming area and fish behaviour (sand sifters , schooling)