Here’s the relevant wikipedia: http://en.wikipedia.org/wiki/Kt_event#Causes_of_extinctions
The discovery of the Chicxculub crater in 1990 and its subsequent dating was when things got more definitive.
But, get with the program! The tidy little narrative of “asteroid hits Earth, dinosaurs all die” isn’t as clear cut as it once seemed. For one, a lot of paleontologists have pointed out that dinosaurs as a group were in decine well before the K-T event (K-Pg in the new time scale), so at best you could only say that the asteroid sped things along.
Also there is the issue with the Deccan Trap flood basalts in India which also date to around the same time, but whose relationship to the K-T event, the mass extinctions and the Yucatan impact are not clear. A lot of people think that the long term climatic consequences of the flood basalts are a more likely cause of the mass extinction than the impact. But, again, the relationship between the Indian flood basalts and the Chixculub crater, if any, is not quite clear. One theory for a while was that the flood basalts had been caused by the impact’s effects punching through the mantle to the other side of the globe, but this has been largely discredited. Another idea is that there was a Shoemaker-Levy esque multi-part impact which might have included an even larger impact in India, but this idea has some dating and other geological issues as well.
So say tuned-- maybe in 10 or 15 years you’ll be asking when did we definitively do away with that nonsense about the asteroid killing the dinosaurs.