The only cockpit indication when using reverse thrust are the moving (reverse) thrust levers.
Toggling reverse thrust, and/or using less then 0% throttle should result in
a white T.R. UNLOCK message being displayed above the N1 gauge.
(Presently there are instead of the N1 and thrust mode reference, 4 com frequencies?)
Once the thrust reversers are in their fully open position, the white message should turn into
a green T.R. DEPLOY message.