Shimano has new models of their groupsets every year but the lines generally keep the same names. This makes it difficult to figure out what year/version a particular bike has. Is there an easy way to figure this out and also determine which components/groupsets are better than others?

For instance, if I have Tiagra derailleurs from 2009 would they be better than Sora derailleurs from 2010 for a road bike?