Couldn't say if this is the way they are supposed to work...but on ours, to engage rain sensing operation:
Switch wipers to second position on stalk - i.e. normal steady wipe for a cycle or two, then back up one step on the stalk. That should be it. You adjust the amount of water required to trigger a wipe by rotating the end of the wiper stalk - rotate further and it's more sensitive. The rain sensor function replaces the intermitent mode and uses the same controls to change it's behaviour.
You could test it by following the above procedure and spraying some water on the screen where the sensor sits....or just wait for some rain
I find on mine that even if left in sense mode, unless I manually kick them into life after starting the car, they won't activate. The Jeep is different - that will always activate if they are left in sense mode.