There is no real rule on how to tell what frame size a gun is, you just kind of have to know.
For example, the M-66 is a K-Frame that is also stainless. The M-19 is the same gun, only blued. The M-586 is a blued L-Frame and the 686 is the stainless version of the same gun. The 24, 25, 27, 28, and 29 are all N-Frame guns, chambered for various cartridges. Put a 6 in front of the model #s and you have their stainless versions.
Come to think of it, there might be a rule, someone correct me if I'm wrong here:
1x/6x are K-Frame (10, 15, 19, 63, 67, 66)?
2x/62x are N-Frame (27, 28, 627, 628)?
5-shot .38 is a J-Frame (36LS, 37)
x86 is L-Frame (586/686)
5-shot .44 Spl. is an L-Frame (396)
.500 S&W Mag. is an X-Frame
Don't know if I'm right on this, but it seems to try to pop out. Can anyone think of anything that breaks my examples?