In a PvP type of test you could relatively easy time the duration a single casting of the IS to determine is level (duration=8 sec + 4 sec/additional level). Spotting the differences in a couple levels of difference in the two arrow skills can be somewhat harder (minior differences in the added fire cold damage and minor difference in the converted variable-physical to elemental damage). Which is easier to tease out without modding the game do you think?
Edit: btw, in v1.09 that 'normal' arrow that was being morphed to a "fire arrow" was more of just a graphics look. The damage was that of a normal arrow shot with 2 to 4 points of fire damage added to it and using the same fire arrow graphic as the other 'fire arrows' elsewhere in the game. Not the improvement that you mistakenly thought it was (converting 50% physical to fire as the Fire Arrow skill did in v1.09).
Edit: btw, in v1.09 that 'normal' arrow that was being morphed to a "fire arrow" was more of just a graphics look. The damage was that of a normal arrow shot with 2 to 4 points of fire damage added to it and using the same fire arrow graphic as the other 'fire arrows' elsewhere in the game. Not the improvement that you mistakenly thought it was (converting 50% physical to fire as the Fire Arrow skill did in v1.09).