Well, here are my observations, in no particular order of appearance:
1. Straight barrel (maybe slight taper) vs. a barrel with a slight "bell" where it joins the frame.
2. Flat cylinder latch vs. rounded latch.
3. Bottom of frame nearly a straight line over the trigger guard vs a "faired" joint of the trigger guard. (Compare photos in Winchester_73's post)
4. Colt Positive Safety vs. no Safety
5. Hawk bill front sight vs a sloped "quick draw"
Hard rubber vs. checkered wood. (I thing only the M1909, M917, and British M1914 had smooth wood.)
6. Finish, bright blue vs. brushed blue.
These are variations I've noticed, have no idea as to dates.