most of the time, it's a small slider on the side of the watch (left side on this vintage IWC)
No rep with this complication yet, but Seagull (I think) showcased their first minute repeater at the last Basel show... so we may see a functional repeater rep soon... who knows...
I love listening to those repeaters and probably the most significant of all complications. Too bad they are sooooooo expensive. Except for some of the electronic ones like the Citizen.