Sometimes you can't taste it at the time of racking. There's a sensory trick that allows you to check for diacetyl that I got from Yeast by White and Zainasheff.
-Pull two small samples of the beer from the fermentor and place in two identical glasses.
-Cover both samples.
-Set one aside.
-Put the other one in a hot water bath that has been heated to 140-160*F.
-Let the glass sit for 5-10 minutes.
-Remove, and using a cool water bath, chill it to the same temp as the control sample.
If the yeast haven't cleaned up the diacetyl, you should be able to smell and taste it in the heated one. Apparently diacetyl is an oxidative product of something else the yeast produce, and the heat speeds up the reaction and makes it more noticeable.
I've had batches that didn't taste like diacetyl straight from the fermentor, but after a few days on tap I start to notice it. Since my last one, I've been doing this D-test every time (if I'm rushing a beer) and it hasn't come up positive but I'll still keep doing it... To me, diacetyl is one of the ultimate let downs after you've put so much effort into making a beer, mostly because you don't notice it until you're all excited to be drinking it.
Not trying to scare you, though - hope your beer turns out well.