They look very similar, that’s true. A handy trick that I found somewhere is to look at where the two small lines go. The lines for kata shi シ would connect to hira し and tsu ツ would connect with つ
Another thing that made me think it’s a “tsu” is the position of diacritic symbols. On “shi” they are often above the line, while on “tsu” they are on the side of it
They look very similar, that’s true. A handy trick that I found somewhere is to look at where the two small lines go. The lines for kata shi シ would connect to hira し and tsu ツ would connect with つ
Another thing that made me think it’s a “tsu” is the position of diacritic symbols. On “shi” they are often above the line, while on “tsu” they are on the side of it