Find out when positive (9)

I believe the answer is:

ascertain

'find out' is the definition.
('ascertain' can be a synonym of 'find out')

'when positive' is the wordplay.
'when' becomes 'as'.
'positive' becomes 'certain' (similar in meaning).
'as'+'certain'='ASCERTAIN'

(Other definitions for ascertain that I've seen before include "Get to know" , "Establish (the facts)" , "Confirm, make sure" , "Establish, find out" , "Verify" .)

I've seen this clue in The Guardian and The Times.

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