how does beatty discover montags green bullet?

Captain Beatty has just finished verbally lashing Montag. Montag hesitates for a few seconds and Beatty discovers the green bullet in Montag's ear. Beatty threatens to track the two-way radio to (Faber) its source.

Beatty keeps at Montag, threating and taunting him until Montag "liqid fires" Beatty. That's it for him!