What do you think of this
https://www.foto-mueller.at/shop/photographic paper-chemistry-lab equipment/slide duplication/kaiser-test-slide set.html
and this
http://www.fotoimport.no/pg08/pg08-2.htm
...?
…
...Or, if, as suggested above, that’s all you’re interested in:
…
carefully scratch a pattern into it. That should normally be enough to judge the sharpness.
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...then see if you can find someone who offers a service called slide exposure. In my student days (less than 10 years ago), you could find such services, for example, in university computer centres and other reproduction facilities. I remember this from my own experience at the FU Berlin and in Mainz, at least. It was simply the precursor to projectors and PowerPoint. I can see that the ZDV Mainz still offers this service:
https://www.zdv.uni-mainz.de/465.php and I’m sure it won’t be the only one.