On the river trip we stop above every rapid and look before we paddle. Here is what the guides are looking for.
The tongue
A smooth V of dark water pointing downstream. That is the deepest, cleanest line. Aim for the point of the V.
Pillows
Water piling up in a soft bump means a rock just underneath. Steer around it, not over it.
Eddies
Calm patches behind rocks where the current swirls back upstream. They are rest stops, and learning to turn into one is the best skill you will pick up all day.