Well, if you are REALLY intersted there are pocket books with this info in them that you can buy at bookstores or commercial aviation websites, usually they have a page dedicated to each a/c type with ways to tell the difference between a 767 and an A-300 for example.

If you see 744 written down that means 747-400, A345 means A340-500, 738 is a 737-800 etc etc.

For the 737 family, the 737-100 and 200 have the long "tube" looking engines attached under the wings, the 737-300 and up have the "tin can" looking engines attached ahead of the wing (for lack of a better description). The -300 is a little larger than the -200, the -400 is larger than the -300, the -500 is smaller than the -300 and with the "next generation" 737's, the -600 = the older -500 only with updated interiors, wings, etc, -700 = -300 and -800 = -400 with the -900 being the longest of them all.

I hope all those numbers didnt toally confuse you