It takes time!
My cousin and I went fabric shopping this week and I kept pointing to machines going "15-90", or "127 Pheasant decal", and she just shakes her head. Can't figure out how I do it.
Here are two that will help you get going:
http://www.sandman-collectibles.com/...r-machines.htm
http://vssmb.blogspot.ca/2011/07/vis...g-singers.html - This is a great visual, but I find that he misses some weird things. Like a 15 is immediately identifiable by the tensioner on the face plate. (It can only be a 15 or a 115 at that point)