For what it's worth, mj12bot.com is even worse. They pull down every wheel every two or three days, even though something like chemfp-3.4-cp35-cp35m-manylinux1_x86_64.whl hasn't changed in years - it's for Python 3.5, after all.
that's not what I meant.
and it is not they, it is it.
i.e. the web server, not bots or devs on the other end of the connection, is what tells you the needed info. all you have to do is check it and act accordingly, i.e. download the changed resource or don't download the unchanged one.
google:
http header last modified
and look for the etag link too.