We have had several (many?) debates on the warm vs. cool pitch temperatures. I feel pretty strongly that pitching warm will cause some off-flavors. Now, I know many feel just as strongly that it won't. I've done alot of reading on the subject, but my own preferences comes from this thinking:
You pitch an ale at 65 degrees to ferment at 65 degrees. You don't pitch at 90 degrees (25 degrees higher) and plan on it tasting as good as it would if you pitched at fermentation temperatures. So, why would I pitch a lager yeast (sulfuric and strangely estery at high temps) 25 degrees higher than fermentation temperature? Especially since it would be a long time before the wort actually cooled to fermentation temperatures if placed into a fridge after fermentation started. The bulk of the fermentation would be done before you'd ever get to fermentation temperature, I'd think. So, what I do is chill my wort to 50 degrees. I have a big, big starter that I fridge and then decant the spent wort. I take it out of the fridge and let it warm to about 48 degrees, and then pitch that in the 50 degree wort. It works great.
Anyway, there is an actual diacetyl test that John Palmer talks about in how to brew (I don't know if it's in the online book, but it might be- it's in the "real" book for sure), but you can also simply taste your beer. If you detect any butter, butterscotch, oiliness, slickness, or even an oily mouthfeel, that's diacetyl. It's always best to do the d-rest on the yeast cake, but can be done later if need be. If you pitch cold, ferment cold, then lager, oftentimes you don't need a d-rest. And some yeast don't produce much of it anyway. If you pitch warm, lower the temp, and ferment it out, you usually need a diacetyl rest.
