Examples from Austen where ‘made sure’ means to be convinced. (I added the bolding.)
In Sense and Sensibility, Mrs. Jennings after Marianne has heard the bad news from Willoughby.
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Aye, I believe that will be best for her. Let her name her own supper, and go to bed. Lord! no wonder she has been looking so bad and so cast down this last week or two, for this matter I suppose has been hanging over her head as long as that. And so the letter that came to-day finished it! Poor soul! I am sure if I had had a notion of it, I would not have joked her about it for all my money. But then you know, how should I guess such a thing? I made sure of its being nothing but a common love letter, and you know young people like to be laughed at about them.
In Emma, Mr. Knightley speaking to Emma about Robert Martin and Harriet.
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The advantage of the match I felt to be all on her side; and had not the smallest doubt (nor have I now) that there would be a general cry-out upon her extreme good luck. Even your satisfaction I made sure of. It crossed my mind immediately that you would not regret your friend's leaving Highbury, for the sake of her being settled so well.
