I remember a captain disagreeing with me wholeheartedly that you don't have to fly at V2 during an emergency turn procedure despite it being written in the manual. Now would this captain have just accepted my words if I were a captain?
I feel as a trainer, you're there to give advice and I always point that out to crews. For some, they feel that I'm criticising them when I'm actually just giving them advice.
As for decision making, it's absolutely correct that I don't have left seat experience and indeed I chose to train because I feel I would gain valuable decision making experience from some great captains. At the same time though, it's clear when a crew hasn't made decent decisions even if one doesn't have left seat experience. Have they gathered all information and applied them to positively identify the problem on hand? Have they communicated with all parties and enabled them to do their jobs accordingly? Are they prioritising their tasks appropriately? Are they even using the correct checklist?
I'm there to observe and if they haven't done or have room for improvement then in the debrief I offer advice and discuss with the crew.