A strip of paper goes into the liquid and comes straight back out with the answer printed on it in colour. No discussion, no second opinion, nothing in between: one dip, one colour, done. English keeps that picture and hands it to people and decisions. Pick the single thing you trust to settle the matter, look at that, and treat the question as answered.
From the word’s history
The packet's Wiktionary entry gives the origin simply as 'From litmus + test': the dye that changes colour with acid or base, plus the ordinary word test. Nothing has been added to that picture; the figurative use is the same test pointed at people and choices.
證據包裡的字源只有一句：From litmus + test。litmus 是會隨酸鹼變色的染料，test 就是一般的「檢驗」。比喻的用法沒有新增任何東西，只是把同一個測試對準了人和決定。
Wiktionary, "litmus test"
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