1. Use spells with similar effects (Blin v Accuracy down): The former was dark and the latter was not (Presumably, earth) using magian staff trials
2. Use element of the spell to determine element (Absorb spells): Absorb-STR worked towards magian trials when used against Scorpions, but not when used against Dhalmels. Absorb-CHR worked against Dhalmels though (CHR=Light, so that makes sense).
3. Determine whether spell element or debuff element determines resistance (Sleep/Absorbs): If debuff determines resistance, then any spell can land the same debuff against the same mob; if spell element, then mobs can resist different spells enacting the same debuff. This is easy to test using dark elements and Absorb-STR. Using Test 1, we know that Absorb-STR is a water based debuff even though the spell element is dark. Dark elements resisted Absorb-STR despite this fact. Thus, we know the spell resistance is not dependent on the element of the status icon itself.
4. Determine the element of terror debuff (Jetturatura): I did a Jettura test (Terror) and found the element was not Dark, Ice, or Earth. I tested this using RDM/BLU with Chainspell + Brew against Dhalmels, Jnun, and Bugards.