Telling the difference between the -300 & -500 is a little tough. The -500 being a little shorter of course. Here's a little tip when looking at Southwest 737's. If you can read the N#, look at the first number. For example if it's N5xxSW, it will be a -500. N3xxSW, it's a -300. This works with the -700 series as well. Keep in mind that not every number will work this way, but it works a lot of the time.
The dead giveaway for telling -400's and -800's/900's is two overwing exit doors on each side of the aircraft.
Hope this helps!
I'm not a real aeronautical engineer, I just play one on Airliners.net.