Larry - A very good question. I asked that question too and the answer I got was the university studies.
I often check several sources online before making a choice about a rust resistant cultivar. I have read comments from others that one listed as high rust resistance was not. So now I question those ratings. But I am also looking for specific pretty faces, so am willing to take a chance on one that is unknown or iffy if I really like what it has to offer in the bloom. Mostly I go with the ones that are truly showing rust resistance. I doubt very few will ever be completely rust resistant, but they may not get a severe attack of rust. Those are the genes I want in my hybridizing program. I also keep reading about Ted Petit's "Enjoy 24/7" daylilies for landscaping are quite rust resistant. Apparently he bred for Rust Resistance over the past few years once rust became an issue here in the USA and especially the south.
