If you really want to train spot,
1. Both 2 & 300 have bloody big wings (comparitive to fuselage length) & winglets.
2. Both have characteristic Airbus nose.
3. As stated the 300 is slightly longer, externally = more windows between first & second exit doors (I think 200 has 12 windows & 300 has more). Look at
www.alirliners.net for more detail.