It’s an art more than a science:
Even with rising inflation, and falling home construction, real GDP contracted at only a tiny 0.2% annual rate in the fourth quarter of 2007, and grew in the first two quarters of 2008, including a quite healthy 2.8% growth rate in Q2.
Not since 1970 has the NBER called a recession for a period including such a strong quarter of real GDP growth (and remember that the 1970 data has been revised substantially in the intervening years). In fact, GDP data were much worse than in recent quarters during 2000-2001 and using them could have led the NBER to date the last recession to mid-2000, not March 2001, and it would have been called the Clinton Recession, not the Bush Recession.