Varies depending on the person & the situation
Calling their name may be quite appropriate
u Lightly tap on the shoulder or lightly placing a hand on their shoulder
Heavy touch/rapid tapping indicates urgency
Ask someone closer to tap them on the shoulder
Wave your hand and arm in the air
Hit your foot on the floor repeatedly or pound lightly on a table
Large groups: flash lights at a slow and steady pace
Rapid flashing indicates an emergency