Diacetel is usually not detected until after bottling or kegging. I have never tasted diacetel in a fermenting or recently finished beer. It develops over time that is why it is a good idea to do a warm rest and not have to worry about it.
From Wikipedia-
It is produced during fermentation as a byproduct of valine synthesis. During this synthesis yeast produces α-acetolactate, which escapes the cell and is spontaneously decarboxylated into diacetyl. The yeast then absorbs the diacetyl, and reduces the ketone groups to form acetoin and 2,3-butanediol, relatively flavorless compounds.
Beer sometimes undergoes a diacetyl rest, which entails elevating temperature slightly for two or three days after fermentation is complete, to allow the yeast to absorb the diacetyl it produced earlier in the fermentation cycle.