verbal cue.

Pitcher says "slider" into the mic. Only the catcher hears the cue in his earpiece. 

Catcher gives the 'yes' sign. 

Pitcher says "inside corner" into the mic. Only the catcher hears through his earpiece.

Catcher gives the 'yes' sign.

The opposition could steal or view in plain sight the catcher's 'yes' sign each time, but would not know that the pitcher said "slider" and "inside corner." It actually makes it more simple, but significantly reduces the risk of stealing the call.