Set marker at current time (works during preview and audio-only preview)
CTRL, 8
Set marker at current time and open marker dialog box
CTRL, OPT, 8
Set and number a composition marker (0-9) at the current time
SHFT, 0-9
Go to a composition marker (0-9)
0-9
Display the duration between two layer markers or keyframes in the Info panel
OPT, click the markers or keyframes