Subtasks can be viewed in either the List View or Page View.
In the List View, if a task has subtasks, there will be a Subtasks button under the task status. Click on it to expand a list of subtasks.
In the Page View, the details window of each task will have a Subtasks tab. A purple numbered box, if shown, will indicate the number of subtasks for that task.
Depending on the user's project permission settings, they may be able to view all project tasks or just a subset. Watchers specifically added to the task will be able to view it.