Why they look similar, I can't answer. NagaiKami might be able to.
But how to tell the difference, that I can do
biggrin I pulled out my fudepen (brush pen) to make it easier

The top two are "so" and "n". if you look at "so" you can see that the "eye" goes down and the "smile" is thicker at the top. This thick part is where it starts. The "n" has the "eye" going across-ish and the "smile" starts at the bottom. This is VERY DIFFICULT to read on a computer, and takes a lot of practice to get used to
sweatdrop The "shi" and "tsu" are almost the same as "so" and "n". "shi" has the "eyes" going across and the "smile" starting at the bottom, while the "tsu" has the "eyes" going down and the "smile" starting at the top.
Did that help any?