757 is unmistakeable due to its distinctive 'dolphin' nose.
As for 320 vs 737, some points:
- the 320's vertical stab is set forward from the end of the fuselage, the 737's isn't
- 320 fuselage rounded at the front, pointed at the back
- 737 fuselage pointed at the front, rounded at the back
- most obvious difference (if you can see them), the cockpit windows. Easier to look at photos as suggested than me trying to put it into words.
hth