If you can see the tail number they go like this:

A-320: N4XXUA

A-319: N8XXUA

The 1 vs 2 overwing exits is probably easier to spot though. Eight vs. 12 seats in F is another.

BTW, Seawolf, the 319 is a SHORTENED 320, not the other way around. The 320 was the "base" or first aircraft in the series so newer versions are derived from it. There is also a really stubby 318 but those haven't shown up in the states yet.