The only way to know if it was calibrated at 12% or 18% that I know of is taking meter readings and compare them with a known meter. I also have the Gossen Ultra spot so I compared them before I bought the meter.
Another way is to take a reading in a clear sunny day at noon and compare with the sunny sixteen rule, but this is less accurate.

So the results are, if you take a reading of an 18% gray card one with your "known" meter and one with the L 508, if the L 508 shows exposures which are 1 stop faster then it is a 12% calibration. You can adjust for this by lowering your ASA, but that is half assed.

As with everything some people have trouble learning how to use it, some not. I was using within 5 minutes after they showed it to me in the store. To me its functions were intuitive and I did not even need to read the manual, but then after using the gossen ultra spot, any metter is easy to use...:P

As a aside note, many people sent their meters back to Sekonic complaning about this, and their response was, "well it is well within standard industry specs" very good customer relations IMO.