Clue: Double alcohol percentage for validation (5)
Answer: PROOF
Explanation: Definition: Double alcohol percentage
triple definition: "Double" as in a proof copy; or used to describe the strength of alcoholic spirits; or how the truth/validity of a statement can be determined 
*edit thanks to KVa: perhaps a double rather than triple definition…
 'proof' is "Double [the value of] alcohol percentage" e.g. 86-proof means 43% alcohol by volume*
 
*further edit thanks to CliveinFrance – the ratio of 'proof' to alcohol percentage is not always 'double', see comment 22