For free and back, I time them from the initial head movement initiating the turn (hopefully it’s down and not up) until the feet leave the wall. For fly and breast, I time first touch to feet leaving the wall.
We also time their underwater dolphins during practice. Between sets, we usually work our turn or underwater. For the underwater, they swim from the middle of the pool to the wall underwater and check their time. They then reverse it.
Since we usually have 3 sets at practice, I do 2 sets of turns and 1 set of underwaters.
Because of time constraints, I don’t have time to record these, but I do spot check. they are also responsible for establishing goal times for these. They monitor themselves. Since they need time to recover, we do these turns on 1 minute interval to allow for plenty of recovery.