The 737 Max-8 has a few features that are distinguishable from the earlier generations of 737s. First, the engines have chevrons at the output, this is the easiest way to tell them apart. Second the wiglets are different, these can be a little more tricky to distinguish as the Next Gen737s also have an option for split wingtip devices.
Avoid flying the new 737 MAX and 737-800 planes from American. Alongside a dismal 30" pitch, they also sport seats that can have diameters as tight as 15.9 and 16.6 inches. While the average seat size you'll get on American is a smidge under 17 inches, try for the older Airbuses or 767s as they tend to have the most cushion coverage.
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