how do you assign a specific organism a trophic level?

Answer 1

Think of what the organism eats.

Eats plants? First trophic level = producer (or autotroph)
Eats things that eat plants? Second trophic level = primary consumer
Eats primary consumers? Third trophic level = secondary consumers

Some examples:

autotrophs = corn, clover, algae, phytoplankton, acorns
primary consumers = person, cow, bluegill, squirrel, grasshopper
secondary consumers = person, bass, dog, robin, toad, mouse
tertiary consumers = person, black snake, great blue heron, barn owl

Notice than an organism can feed on several different levels.

Answer 2

what does it eat and does it get eaten. easy as pi. Its more of a common sense thing than actual calculations. Just find where an animal fits best.