I agree with Denny's comment. As a result of the stroke order, I find the lines steeper in ツ/ソ (tsu/so) compared to シ/ン (shi/n), which is a bit flatter.
In handwriting, some people write with an extra 'hook' at the starting end of the stroke. If you do a google image search for "handwrite tsu japanese" you should see some examples.
Also, because katakana is often used for loan words (many English words), you can sometimes guess which character is in the word if you can't differentiate them.
Hope this helps!
ツ "tsu" the bottom line is a downward stroke and シ "shi" - the bottom line is a upward stroke
the same with ン "n" the bottom line is a upward stroke ソ "so" the bottom line is a downward stroke.
one way to remember is ツソ(t(so)on) we will be arriving and (shi)(n)kansen is taking off
Check out the "Obenkyo" app, it will show you the stroke orders
hope that helps