How many letters does the top answer for “the act of validating; finding or testing the truth of something” have?

The most common answer is substantiation (14 letters).

Crossword clue: the act of validating; finding or testing the truth of something

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

substantiation 14 letters wn-def

the act of validating; finding or testing the truth of something

validation 10 letters wn-def

the act of validating; finding or testing the truth of something

the act of testing something 50% similar → trial