Poose,
Jets don't do pre take off power checks. If they did, imagine the jet blast of a 747 on the next A/C in line at the holding point. No room to turn 45 degrees.
If a reverser is unlocked then a warning will appear. (Assuming the correct maintenance procedures were applied)
If during take off or flight a reverse unlocked indication appears then the immediate action is to close the power lever of the affected engine. If below V1 then Stop.
The Lauder Air crash on 26 May 1991 was due to an un-commanded thrust reverser deployment in flight. The situation was un recoverable after less than 10 seconds.
I've had a reverse unlock indication after start and that involved shut down and maintenance action to lock out the reverser before dispatch IAW the MEL.
Hope this helps,
Dixi.