It is not easy for the average DIYer to jack up the Tesla rear and safely crawl under it to do this check. Here is a relatively easy way to raise high enough for this check https://teslamotorsclub.com/tmc/posts/7255181/ and https://teslamotorsclub.com/tmc/posts/7014538/ and read subsequent post on #61/#62. 2 Jacks simultaneously on both sides would work followed by placing wood blocks/ramps under the rear tire. One concern is the car is very wide and often not easy to get floor jack perpendicular to the side of the car where jack's wheels can roll. Consequently, there are likely compression forces as the car gets jacked up. I jacked up one side at a time once. Placed first side tire on wood (has good friction, doesn't slide) and jacked up the other side with jack at ~30 degrees parallel to side of car. Heard a loud ping sound. was probably my jack but don't take chances. Anyway, drive up shallow ramp method is probably the easiest if tires don't need to be off for checking speed sensor.
ajbessinger et al @ TMC
QC Charge @ youtube
Tips
Avoid disconnecting the connector to the speed sensor. The snap fit clip seems easy to break. Just remove the bolt that secures the speed sensor and pull it out with your fingers. There is a sealing o-ring so gently insert a little and wiggle all sides under it comes out. Any leak will be immediately evident, there is no need to pull it all the way out (only good for pictures). Here are pics of the sensor with the o-ring for reference.
How to let an expert anywhere in the world diagnose your car @ youtube (Toyota example, but applies to tesla / mercedes too).
This simple test can provide a positive but not a negative. If you have a wet speed sensor, you may also have inverter coolant intrusion. Probably easy enough to do while the car is jacked up for speed sensor check (but note the HV caution). https://teslamotorsclub.com/tmc/threads/diy-tesla-drive-unit-repair.274163/page-2#post-7053940
Some cases involving a coolant leak result in soaking the motor windings, in turn causing an "isolation fault". The car stops running and the stator needs to be dried out for long periods under high heat. An insulation tester ("Megger" / "megohmmeter") is required to confirm that the isolation fault is gone. Source. Some inverters gets damaged by coolant. This requires sourcing a salvage inverter and reprogram (or complete LDU replacement of course). see also here and here.