Synonyms for «proof something works»

Synonyms for individual words can be found below.

He doesn't promote anything unless he's seen scientific proof it works or has used it himself. (hamptonsrealestateshowcase.com)
If you are really convinced to follow someone else's lead, get proof it works. (lawyerist.com)
Will a W - 2 be sufficient if someone wants proof I worked there? (careerdigital.com)
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