Why do シ and ツ look so alike?

Both are two short strokes plus one long sweep. They come from different kanji (シ from 之, ツ from 川), but they ended up with the same parts, so only the angle of the strokes differs.

Which one is shi and which is tsu?

シ is shi: the two dots are stacked on the left and the long stroke rises to the upper right. ツ is tsu: the two dots sit side by side on top and the long stroke falls to the lower left.

What about ソ and ン?

The same rule. ン (n) has one flat dot and a long stroke that rises to the upper right. ソ (so) has one upright dot and a long stroke that falls from the top to the lower left.

What if handwriting is messy?

Native handwriting is often loose too. Read the whole word: a loanword such as ツナ or パン is easy to guess from context, and that is how most readers settle it.

シ vs ツ and ソ vs ン: how to tell them apart

シ (shi), ツ (tsu), ソ (so) and ン (n) look almost identical, and they catch nearly everyone who has just finished hiragana. Here is one trick that works from the hiragana you already know, two backup clues, and a short test.

Four cards pair each hiragana with its katakana: し with シ, つ with ツ, ん with ン and そ with ソ. A red arrow traces the last curve of each hiragana and the long stroke of each katakana. シ and ン rise to the upper right; ツ and ソ fall to the lower left. — Trace the hiragana. Its last curve points the same way as the long stroke of its katakana twin.

The trick: where does the long stroke finish?

Each of the four ends on one long stroke. In シ and ン it starts low on the left and rises to the upper right. In ツ and ソ it starts at the top and falls to the lower left.

Your hiragana already show it. し and ん both finish by sweeping up to the right. つ ends curling down to the left, and the diagonal at the top of そ falls the same way as the long stroke of ソ.

The four katakana, how many strokes each has, and where the long stroke goes
Katakana	Reads	Strokes	Long stroke	Hiragana twin
シ	shi	3	rises to the upper right	し
ツ	tsu	3	falls to the lower left	つ
ン	n	2	rises to the upper right	ん
ソ	so	2	falls to the lower left	そ

Two backup clues: the dots

シ vs ツ. シ has two dots stacked one above the other, lying fairly flat. ツ has two dots side by side on top, standing more upright, like two ticks above the long stroke.
ン vs ソ. ン has one flat dot in the upper left, and its long stroke starts low. ソ has one upright dot, and its long stroke starts high, right beside it.

Is it really the same shape as the hiragana?

Partly, and it is worth being honest about. し and シ both come from the kanji 之, つ and ツ from 川, and そ and ソ from 曽. But ん comes from 无 while ン comes from 尓. So treat the trick as a memory aid for the direction of the stroke, not as history.

Sources: Wikipedia, Shi, Tsu, N, So (kana).

Test yourself: read these six words

Say each one in your head, then open it to check.

ツナ
tsuna — tuna
シーツ
shītsu — bed sheets
ソファ
sofa — sofa
パン
pan — bread
シーソー
shīsō — seesaw
ツアー
tsuā — tour

Practise them on paper

Writing the long stroke the right way is the fastest fix. Print free katakana practice sheets with stroke order.

Questions people ask

Why do シ and ツ look so alike?: Both are two short strokes plus one long sweep. They come from different kanji (シ from 之, ツ from 川), but they ended up with the same parts, so only the angle of the strokes differs.
Which one is shi and which is tsu?: シ is shi: the two dots are stacked on the left and the long stroke rises to the upper right. ツ is tsu: the two dots sit side by side on top and the long stroke falls to the lower left.
What about ソ and ン?: The same rule. ン (n) has one flat dot and a long stroke that rises to the upper right. ソ (so) has one upright dot and a long stroke that falls from the top to the lower left.
What if handwriting is messy?: Native handwriting is often loose too. Read the whole word: a loanword such as ツナ or パン is easy to guess from context, and that is how most readers settle it.