UniTed,
To distinguish the 300 between the 500, you just need to look closely and you'll see the 500 hundred looks stubby, more like a squeezed 300. Think the 747 vs. the 747SP
To distinguish the 737 classics from the
NG's, what I do is look at the engine pylon. The engine pylon on the classics is straight and thick, while in the
NG, the pylon is thinner, curved and more "delicate" This is my method, I came up with it after hours of comparing pictures. Some others may disagree with this but it works for me.
Regards,
AR385