If ever I'd seen one If I ever heard one Those are both variants of 'If I've ever seen/heard one'. AmE in particular tends to use 'ever' with simple past rather than present perfect. I'm not sure whether you can classify a whole phrase as an 'intensifier', which is usually limited to such words as 'very' and 'hardly', so you probably won't find it listed as such, though that is the sort of purpose it serves.
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