From Longman Dictionary of Contemporary EnglishI ought to knowI ought to knowKNOW somethingused to emphasize that you know about something because you made it, experienced it etc ‘Are you sure there’s no sugar in this coffee?’ ‘Of course. I ought to know – I made it!’ → know
Examples from the Corpus
I ought to know• I've had nine, so I ought to know.• Christmas is coming and he thinks I ought to know about some little joints called Tiffany's and Bloomingdale's.• "Are you sure there's no salt in it?" "Of course. I ought to know, I made it."• But if it meant this I ought to know it. 