The NG aircraft are slightly higher off the ground, so the engine cowlings are rounder looking, especially as seen from the front, and not the flat-bottomed versions you see on the -300/-400/-500 to maximize ground clearances.
The NG aircraft have longer wingspans, and the vertical stablizers are also a little longer.
The NG also have differences in landing lights than the older -100 through -500 versions. All 737s have a landing light on each side where the wing meets the fuselage. There's another landing light on each side of the aircraft too. On the older 737 versions, this "outboard" landing light is built into one of the trailing edge flap fairings, and the lights are just a few feet inboard of the wingtip. On the NG versions, there is on "outboard" landing light there, but you'll see them having been relocated to the underside of the fuselage, just either side of the centerline. Thus, all 4 appear much closer together.
Southwest also has the flap fairings on their -700s painted red, compared with the standard gray on other 737 versions.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.