Well, if it's an "original" (-100 or -200) you'll see the engines are the older turbojets, cigarlike, tucked up under the wing, and the tail will hit the fuselage with a slight curve.
If it's a "classic" (-300, -400, or -500) you'll see the engines right in front of the wing with a noticable flattening on the bottom and the tail will hit the fuselage with a big straight diagonal section in front.
If it's a "new-generation" (-6, -7, -8, or -9 00) you'll see the engines a little rounder, the plane a little taller, and the wingspan very much increased. It can be very hard to tell a -700 from a -300, but -800s usually stand out for their length and -600s for their shortness in contrast to the huge size of their wings (-900s with be easy 'cause they'll all have scraped up tail undersides).
As far as separating -100s from -200s, you won't see any -100s so it's simple.
As far as separating -300s from -400s from -500s, -400s have two overwing exits, and -500s are really short. You can count windows if you really wanna' know.
And like said, -600s are really short with huge wings, -700s look like -300s with rounder engines and longer wings, -800s are really long, and -900s are really really long.