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.