As soon as the lock starts to enter, you can consult the lock plan and the position of your ship in the lock.
By pressing on the gray field with your lock reservation(s), you will enter the overview with your lock reservation(s). Press here on the 'View Locks' button. If you have two lock reservations, press the button at the lock for which you want to see the lock plan.
Or you can also just press the voyage field and access the lock plan via the 'Locks' tab.