Cockroaches can be trained like dogs! Sort of…

In 2007 Japanese scientists trained cockroaches to salivate on cue, much like Pavlov’s dogs. Instead of a bell, the bugs were exposed to the scents of peppermint and vanilla just before they were fed. The cockroaches learned that the scents equaled dinner and salivated, even if no food was in sight. Just like the dogs and the bell.

My question: how do you tell when a cockroach is salivating?