You have the 722.9 "7Gtronic" 7-speed automatic, and it not only lacks a dipstick, it doesn't even have a filler tube for using a dipstick (like the 722.6 does). The 722.9 fluid level is checked from underneath. Inside the pan, above the drain plug, there is a vertical overflow tube. Fluid is pumped in through the drain port, and then the level is checked at 113F so that it is high enough to overcome the height of the overflow tube and drain out. Once it drains to a slow trickle, it's full.
My advice is that if you think your fluid level is low, you should probably go ahead and do a fluid and filter change. MBZ officially recommends a single F&F change at 39K miles for the 722.9, but common opinion, based on observations of fluid quality, is that you should do it every 39K miles.
The service will cost around $400 at a dealer, a little less at an indy. If you are mechanically inclined, you can DIY, but it's complicated. You will need some special tools, like a filler pump, but even buying everything you need, you'll still spend less than having it done, and then you'll have the tools for the next change. Probably the most difficult part is being able to lift the vehicle enough to safely work underneath it, with the engine running and with the transmission pan perfectly level. In other words, you probably don't want to do this in your driveway, but if you have a flat garage, and a good jack and solid jack stands (like I used) it's not a bad job. It took me 4-5 hours the first time, but I could probably do it under 3 now.
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