One Kirimon: What shape is the coin?
Square
Two Kirimon: Does it have the character 一 or 二?
On the same side as the kirimon, look at the right edge, in the center. You'll see one of these two characters between the kirimon.
一: Are there any characters in the top obverse corners?
Two extra characters
Double Hon (両本) Keicho Ichibuban
One Character: Which character is it?
本 in grass script, commonly mushy
Single Hon (片本) Keicho Ichibuban
元, usually mushy
Genroku Ichibuban
乾, usually mushy
Hoei Ichibuban
佐
Sado Ichibuban
文, usually mushy
文 in grass script
正
Ansei Ichibuban
二: Are there any characters in the top obverse corners?
No: Does the 分 slope up?
Look at the top right stroke of the 分 character. The bottom of the stroke may bend slightly up, or be straight/bend down.
No, it's straight or slopes down
Slopes up: Is there a V at the bottom of the top kirimon?
Look where the kirimon meets the semi-circular border. Some have a small V connecting the two; others just have a space. If you're having trouble seeing it, there are more pictures here.
Yes: Which character is it?
文 in normal script
文 in grass script