ツ --> this means 'tsu' in the katakana system シ --> this means 'shi' in the katakana system If ur unable to distinguish them, then シ think of this characters as " When she(shi) is happy she looks up in the sky" as you can see シ is kinda looking upwards.
Practice writing them (the Quick Draw game in Renshuu is great for this!), and try imagining them superimposed over their hiragana forms while you do. The direction the pen moves as you draw the strokes in these characters is the same direction the pen moves when writing the hiragana form -- the direction of movement affects the shape of the final character, so by drawing them you'll develop an intuition for which one is which.
し → シ → strokes are drawn counter-clockwise, and the position of the dots in the katakana roughly line up with the stroke of the hiragana (along the left side of the character)
つ → ツ → strokes are drawn clockwise, and the position of the dots in the katakana roughly line up with the stroke of the hiragana (along the top side of the character)
adding to the other great responses the way i started distinguishing them is シ being smirk face always with eyes not straight and ツ a genuine smiley face
Hi, maybe looking at katakana words can also help because of the context. For example if you know that スポーツ means sports, the ツ should be tsu if the word sounds right. Or if you know シングル is single, the シ can only really be shi.
Also, only one of them has a small form. So if you see something like サッカー シリーズ (soccer series) the small one has to be ツ tsu, so the other one has to be シ shi. So context and elimination of options.
What helped me quite a bit with katakana in general was reading through this list: https://ja.wikipedia.org/wiki/... Scroll down a bit to the table with the country names.
Famously, this is one of the pairs of characters which get confused with each other easily. There are some tells.
シ = し ツ = つ
A prominent tell with シ is the katakana's 2 marks are higher above the curve, visually shaped like し with it's long vertical stem above the curve, basically.
Naturally, ツ is the opposite. The upper part of the lines are as high as they can go, forming a shape like a tsunami wave ought to be, with the curl of the wave.
By themselves, they can also be hard to read with certain fonts. The next easiest thing to notice is which angle the two upper marks position themselves. Failing that, if they're used in spelling, try to read the whole word. Chances are, you're gonna think of the one that makes the most sense. If you find the small form 「ッ」in the word, then it's no question it's the small 「っ」.
The more practice you have with it, and seeing it in different fonts, the more you're likely to read it better without straining to think much to fill in the blanks. When you learn a lot of words that use these characters, it's much easier to see what it says knowing the whole spelling.
