This is a conceptual question and has probably been asked before in many guises and forms. Mine has a little twist to it.
I realize the Hubble Law tells us the farther back in time and space we look the faster the universe appears to be receding from us and the more of a red shift is observed...after all we are looking back and getting closer to when the universe first began the acceleration.
It normally would be accelerating faster farther back in time so how can one distinguish between that acceleration and what is happening now?
They only thing I can come up with is that acceleration between us and a closer "spot" in space is faster then can be explained given the acceleration that is observed much farther in the past. In other words the acceleration as you look farther back in time is actually slower than expected as compared from us to closer locations.
So the standard candles are the mechanism to get at the acceleration from us to a "close" object. Comparing the expansion rate given by Hubble the closer object is moving faster than it should given the speed of the object farther back in time so we conclude we are speeding up relative to the past.