How can we know if a country is in recession from the given graph?

Question

but if i have a zig zag curve of real GDP and i can see that during 1990 and 2001 the real GDP is decreasing so from both of them which one should i use that will tell me when the recession occured. will it matter if one is below zero

Answer 1

If equilibrium output (GDP) falls for more than two quarters (sometimes months) below normal level.

In case if your graph shows time series of real GDP growth then you should check if growth is negative over certain period.

http://en.wikipedia.org/wiki/Recession

Answer 2

I can't see the graph.  If it is the AD-AS graph, then a recession is implied once you have equilibrium in the Keynesian range, which is the flat/horizontal range.