Lexical stress is contrastive in English, but when asked to prove that it is, we must be careful. True stress minimal pairs are lot rarer than a lot of people think. By “true stress minimal pair” I mean a pair of words, which mean different things, and differ only in the place of primary lexical stress. An example is insight ˈɪnsaɪt vs incite ɪnˈsaɪt.
The problem is that the stress status of a syllable often determines what vowel occurs in it. Take, for example, the two words export (noun) and export (verb). Yes, they differ in the place of lexical stress, but in many accents, including GBE, the vowel in the first syllable of the noun is e, but in the verb it is something else, ɪ or ə.
Many of the “true” minimal pairs are like import (verb)~ import (noun), where one member is a verb and the other a non-verb (noun or adjective) of related meaning. The non-verb has earlier stress than the verb: so ɪmˈpɔːt vs ˈɪmpɔːt. I don’t think I have ever come across a comprehensive list of such minimal pairs. Maybe you know of one.
Here are a few others I have thought of. Additions welcome.