acid test 
noun [in singular]
A conclusive test of the success or value of something: gritstone is the acid test of a climber’s ability...
- When you apply the acid test of value for money, hardly any make the grade.
- It can be used as an acid test of whether a person has interest in knowing or may be blinded by ideological convictions.
- And that, according to any number of people here, is the acid test of the world's new-found determination to stamp out doping.
Origin
Figuratively, from the original use denoting a test for gold using nitric acid.
- lay something on
- lay something on the table
- lay something on thick (or with a trowel)
- lay something out
- lay something (to) waste
- lay something up
- lay store by
- Layton
- lay up
- lay waste to (or lay something (to) waste)
- laywoman
- lazar
- lazaret
- lazarette
- lazaretto
- Lazarist
- Lazarus, Emma
- laze
- lazily
- laziness
- Lazio
- lazuli
- lazulite
- lazurite
- lazy