The acid test

The acid test :

A situation or event which finally proves whether something is good or bad, true or false, etc.

The original use of the phrase was to describe a method of testing for gold with nitric acid (gold being resistant to the effects of nitric acid).

1990 Which?: These deals are designed to encourage impulse buying, so the acid test is whether you would have bought anyway.

Come the acid

Be unpleasant or offensive.
Speak in a caustic or sarcastic manner.

Put the acid on someone

Try to extract a loan or favour from someone. (Australian & New Zealand informal)

The acid test :

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