an acid test

Meaning: An acid test is something that shows the true worth or value of something or someone.

For example:

• Our new player has been doing well at training and in practice matches, but the acid test will come in her first real match.
  
• The new political party is doing better and better in the polls, but the acid test will be the next election.
  

Origin: Possibly related to the fact that a test involving acid has been used for over a century to prove that a metal that looks like gold is, or is not, really gold.