Position of the hands
This is the most basic and most obvious – whichever side the boat is down to needs to raise their hands. If the boat is down to port then ports need to raise their hands and starboards need to lower them. If the boat is down to starboard, the starboards need to raise their hands and the ports need to lower them.
Unnecessary movements or over-adjustments
Once the boat’s achieved a good balance, point it out so the rowers know that this is where they should be carrying their hands. Making a lot of unnecessary movements throughout the recovery, not having control over the handle, or just over-adjusting will make it difficult for the boat to level out and can result in overcompensation from the other side, which in turn makes it hard to figure out if the boat set up because the right adjustments were made or because someone is compensating by adapting their technique.
Not moving the hands through a horizontal plane
This applies to both the drive and the recovery and is easily noticed because the blade will either be up in the air (i.e. the rower is skying) or buried too deep (i.e. digging). A good visual cue to help fix this is to tell the rowers to carry their blade level with their oarlock. This might be easier for you to see from the front than it is for them from the side so make sure you point it out once they’ve made the adjustment.