It is the area forward of the rear entry door and aft of the forward entry door where the fuselage sections are different between the 2.
If you are close enough to see the cargo doors - located starboard, note that the 319 abuts the forward wing root to fuselage interface while the longer A320 has about 1m spacing between. On the tail end, note that the shorter 319 has only 5-6 windows before reaching the aft edge of the cargo doors while the 320 has 7 windows.
If the paint (or your distance) obscures the cargo door edges, or you are looking at the port side then you can count the number of windows between the FWD door and the forward wing root to fuselage edge. At least 8 are on the 320.
Simplest way: - so why did I wait till last for this one......
At the aft end of the fuselage, the underside curves forward (and downward) in a sort of scallop on the 319, while the A320 lines in the same area level out a bit between the curves and their transitions.
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