You have to do the work. There are tons of info out there, just start to compile intro dates, first look reviews, Shimano docs, etc. and build yourself a small data base of the groups you want and the years produced. If you are going back 10/15 years it can't be that tough.
And since a picture is worth a thousand words, get familiar with how the different groups look so you can look at a bike and know it has this or that series.
There is nothing specific to Giant. Manufacturers used stock on hand or ordered; other than for those transition years when one 105 or Ultegra groupset was used and also a possible a later series in the same year.
John