Perhaps officially introduce an additional role for the senior F/A. "Check slats and flaps". By some designated point in the taxi out, if the slats and flaps aren't visibly set then notify the flight deck. This takes the onus off aware passengers and removes any stigma involved in the F/A "telling the pilots how to do their job" or for that matter the aware passenger/s telling the cabin crew. After all, the flight crew already tell the cabin crew to "arm door and prepare for take-off" so why not the other way around as well?