Acid test

[H]e reflected that few professional beauties could have stood, as this woman did, the acid test of that mercilessly brilliant morning.

Source: wiktionary

"Eggs, Parker," said Freddie solemnly, "are the acid test!" / "Yes, sir?" / "If, on the morning after, you can tackle a poached egg, you are all right. If not not. And don't let anybody tell you otherwise."

Source: wiktionary

The acid test: will the British public abide by such stringent guidelines about where they should stand, walk or sit in our stations and on our trains, when they have been used to a turn-up-and-go railway where they enjoyed freedom having bought their ticket?

Source: wiktionary

This was the acid test – at least before Sunday's final with formidable Italy – and they came up with the goods not only in the context of Euro 2020 but also in demonstrating their character and big match mentality.

Source: wiktionary