The old generation 737-100 and -200 series have the long "cigar-tube" engines that run under the wings. The newer generation 737s can be distinguished from similar looking Airbus models by looking at the base of the tail. It has a kind of an extra triangular part that joins the fuselage and leading edge of the tail. Also, next gen 737s have engine nacelles that look somewhat "flattened" on the bottom, but you have to see this from the front.

FewMiles..

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Unofficial Guide to AAdvantage: http://home.webflyer.com/fewmiles/AA/