There are three generations of 737's.
1. 100 and 200 series (late 60's and 70's)
2. 300 thru 500 series (80's)
3. 600 thru 900 series (90's)
All nine versions have the exact same cockpit windows and fuselage width, exactly like the 707 and 727.
The oldest 737's, 100 and 200 are easily recognizable because of the engines and the tail: the engines are skinny Pratt & Whitney and extend in front and behind the wing, the tail doesn't have an angled tail fin like the tail on younger 737's.
Now, between the other two generations of 737's, it's hard to tell. It's more difficult to make the distinction because they all have CFM engines and an angled tail fin. I have to look closely and concentrate to tell which 737 it is, whether it's a NG737 or not. I know that the NG737's have a higher tail, larger wings and less flat engines. Of course I can tell quickly if it's a between a 300 or a 700, between a 500 or a 600, between a 400 or a 800 because of the length (the 400 and 800 have two overwing exits on each side). Southwest painted the flap canoes in red on their 737-700's to make it easier for ground crews to tell which 737 it is.
The first time I saw a Next Generation 737 in real was three months ago in Newark, it was a 800 of Continental. I could tell because the tail was high and there were two overwing exits on each side, I knew it was not a 900 because that one doesn't exist yet.
When I look into the cockpit of a 737, I can tell which generation of 737 it is.
PS: Who knows, maybe there'll be a 737-1000!