Answer
The researcher could damage a leaf mechanically, without using an insect, and see if insects still skipped to further-away leaves to begin feeding. This would indicate that the skipping was not due to insect choice but to changes in the plant due to damage.
Work Step by Step
There are multiple tests which could be suggested, but all would need to separate the damage from the insect's role except in choosing the next leaf or, if the chemical released by the plant were known, that could be applied to some leaves to see if insects did not eat them.
