Saw Guerrera used a creature named Bor Gullet to verify whether Bodhi, the Imperial pilot, was telling the truth about Galen Erso.
The creature attached its hand-things to Bodhi's head and supposedly read his mind, and supposedly verified what Bodhi said (also leaving the latter a bit confused).
The problem is - how did Bor Gullet communicate to Saw Guerrera? How would Saw Guerrera understand whether Bodhi was telling the truth or not, given that Bor Gullet doesn't seem to be able to speak? Would his eyes light red or green, or something?