So it's kind of like the difference between a capital "I" and a lowercase "l". It's much easier to tell the difference between a lowercase i and a capital L, but in context, you can read this sentence perfectly despite that I used both "I" and "l", which look very similar by themselves.
There are some fonts and handscripts where you'd find a tiny difference in the edges of the kanji for ichi, just like you can find serifs on some english fonts, but on common handwriting and bold fonts like these ー and 一 are almost identical... So you'll depend on context. But isn't hard to notice it once you get more used to it