ー: いち or prolonged sound?

[readingrains]

Hello how do I tell if a dash is supposed to represent the kanji for “one” or used as a prolonged sound for the preceding kana? Is it just an experience thing?

[Belthazar]

Context. Generally, you’ll find kanji with kanji, and katakana with katakana.

So, 一体, 十一円, 一石二鳥 are all the kanji for “one”

コンピューター, オーストラリア, ジョージ are all the katakana long vowel mark.

[rodan]

Also, if the writing is vertical, ー is rotated, and 一 is not, like in this random example from a magazine:

一方 uses the kanji 一, and kanji aren’t rotated in vertical text,
パートナー uses the katakana notation for a long sound, which is rotated in vertical text.
(but the context tip-off still applies too, of course)

[morteASD]

Adding to the above, the kana is also shorter, which is relatively easy to tell from the kerning.