Possible Clues:

• A severe or conclusive exam or trial
• Conclusive moment (using litmus paper?)
• Searching examination - involving litmus paper?
• By which one proves cat's diet is poor?
• Severe trial; old assay method
• Conclusive experiment for which litmus paper is needed?
• Crucial assessment dictates changes
• Dictates to undergo conclusive trial
• Quiet level for actor
• Goldsmith's check on account that's concealing heart of metal