Like Richard said, obtain the "testing" dipstick. you can buy the actual Mercedes stick or buy a clone on FleaBay

722.6 transmission fluid dipstick in eBay Motors | eBay

Have your codes read, or buy a cheap Chinese reader-It is not OBDII reader..
Here is a how to get a reader, see post 11, and see the diagnostic data on post 7

99 S320 Transmission Rebuild - Mercedes-Benz Forum


Best of luck,

Martin