OK, here's the deal: MB changed where they put sensors on almost every year and it differed between the models. So, simply look for the sensor wire that is quite obvious as it comes out of the caliper and connects into another plug. That will tell you if there are sensors on that wheel. It's safe to say that there is always one on the front. Either way, if you pull a wheel off to look for the sensor wire, you can also check the pads closely. You haven't mentioned how many miles are on you pads, but I've gone thru 2 sets on front before the rear were replaced.
PS. There is plenty of discussion here on replacement pads. The mintex reduce the dust, but I couldn't get a good reproduction of OEM for the rear. Whichever pads you pick, you need to replace the rotors too (usually).