It's really hard to miss the supercharger not kicking in, you'd know if you were afflicted by it. For me, sometimes it'd get up and go when I put my foot in it and other times it'd sit back (the RPM would shoot up, it'd feel like it was going to go) but it just didn't. I believe that when I started noticing this it hadn't gone out completely, as it seemed extremely sporadic even in a short period of time that I don't believe it could have cooled down in. I noticed it most on the interstate and tested it there, got up to about 70 mph and planted my foot... it'd go from 70-100 really quickly then I'd do it a few more times and eventually the supercharger would fail to kick in... it did get progressively worse. If you're having these issues be sure to check the fuse, #13 on the SRT-6 but I'm not sure what it is on the SLK32. If you really want to find out if it's working, feel to see if it's running. Take the car out and get it up to 2500 RPM, the intercooler pump stays on after this point until the car is turned to the ON/OFF position. I'm not sure where the pump is on the SLK32, I believe it's behind the front passenger tire well but it looked harder to reach than the SRT6's.

Edit: RL67037 beat me to it by a long shot! The pump seems to be the most common issue, sometimes the fuses blow and this should always be the first assumption as it's the cheapest, and every now and again it'll be the thermostat. There are NO WARNING LIGHTS OR CODES.