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An easier way to recognise the 737-900 (apart from it being very long) is to look for the "squashed" engines. The 737 is close to the ground compared to the Airbus - a throwback to the days of the 737-200 with its low bypass engines.
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Just be a bit careful on the "squashed engine" thing. The original CFM-56 737s had even more squashed engines, ie. the 737-300, -400, -500. The NG series of 737s (-600, -700, -800, -900) have slightly less squashed engines and can be distinguished by this feature with great care with a good view.
However, the real spotting challenge remains distinguishing the 737-300 from the 737-700, the 737-500 from the 737-600, and the 737-400 from the 737-800 / -900.
As always with spotting aircraft types, it helps to know which carriers operate which types, as this narrows down options considerably, ie. easyJet do not operate A320s, so all their Airbuses are therefore A319s, despite the double overwing exit doors..... But I grant, distinguishing a KLM 737-400 or -800 or -900 is often tough.
In other respects, distinguishing the A319/A320/A321 is pretty easy when you get the exit door configurations understood. The A318, though, is a little tricky vis-a-vis the A319 - look for the taller vertical stabilizer.