I work backwards from the slickness to the taste, as it is often a bit subtle to my palate. I am not blind to it, but I have to really search when a paired judge finds it based on flavor alone. That said, I have had judges in competition say they detect diacetyl and if I don't readily find it (since I am judging with them as well), I will ask a very experienced and diacetyl-sensitive judge (if present) to confirm. Surprisingly, the initial judge is frequently picking up something other than diacetyl.
I had a beer that was diacetyl charged that I brought to a club meeting for comment and the club member that holds an annual tasting class confirmed it and asked for a few bottles for use at the next tasting exam, which I provided (bottled off tap). In the interim the diacetyl was re-absorbed, evidently, because the flaw was gone by the time the tasting exam was held. Odd, but true.