Perhaps our rostering software is somehow unique in this, but crew still in the training phase will have a (T) added behind their names in the crew list. And line trainers acting as instructor on that specific leg will have a capital i (I) added to indicate they are instructing. It is of course only as foolproof as the newest fool in crew control, or flight crew paying attention to those details, but still.
Very easy to see if flight deck crew is new, or to take some extra time into account if they are performing training in the cabin etc.