Here's how I would do it on that car. In your garage, where the windshield is clean and dry, turn the wiper switch to the first position; the wiper will go once. Then wait, if it doesn't wipe again - you have a rain sensor.

On the w210 (like my 2002 E430) - the rain sensor/automatic mode replaces the 'low' setting on the intermittent stalk (though the switch itself doesn't look any different').

With the switch in the 'low' or 'auto' position, the wiper will still wipe one time (regardless of if there is rain or not) - but it will not wipe again until the sensor detects the rain on the windshield.

__________________
Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE