The transport controls are located in the top part of each document frame. From left to right you see the Rewind, Stop, Play, Fast Forward button, and the Loop toggle. Keyboard shortcuts are Space or Numpad-0 for Play/Stop, Numpad-1 and Numpad-2 for Rewind and FFwd, Numpad-/ for Loop toggle. Shift+Space will play back at half speed, Shift+Alt+Space will play back at double speed.
Loop playback: the loop playback uses the current timeline selection and can be altered using the timeline axis, the timeline pointer tool or the observer time tab.