Telling シ, ツ, ソ, and ン apart
Before guessing a whole word, inspect the stroke arrangement in similar-looking katakana. Compare print and handwriting so that recognition does not depend on just one font.
Compare シ and ツ
シ and ツ both have three strokes, but the short strokes are arranged differently and the long stroke has a different direction. In standard handwriting, look at シ's short strokes on the left and its long stroke rising toward the upper right.
For ツ, notice the short strokes near the top and the long stroke moving downward. Angles vary between fonts, so do not rely on a single rule such as identifying シ from the tilt of just one short stroke.
Check direction in ソ and ン
ソ and ン have two strokes. In standard handwriting, ソ's long stroke moves downward, while ン's rises toward the upper right. Observe both the starting point and the balance of the whole character.
ン represents a mora in Japanese and cannot be substituted for ソ. Read the final character of パン (bread) and the first character of ソファ (sofa), then check whether you can recognize them in isolation.
Return to whole words
シャツ contains both シ and ツ. Compare the letters, then read シャ・ツ. Include the small ャ so you do not split it into the wrong rhythm as シヤツ. Also distinguish ツ in カツ丼（かつどん） from ン in 揚げパン（あげぱん）, where katakana appears alongside kanji.
After finding a reading error, write the mistaken character beside the correct one instead of only copying the answer repeatedly. Next, read the same word in another font to check whether you truly recognize the distinction.
Check: Are シ and ツ in シャツ the same character in different sizes?
Show the answer and why
No. シ and ツ are different katakana. シャ combines with small ャ to make one mora, while ツ contributes another mora.