Maybe something along the lines of "do you do a senior sendoff? My (possibly fictional) next door neighbor who's in [insert sorority not at your school] just graduated and she was telling me what a special thing it was." If they say they don't have enough seniors for it to have a point, that would be a hint that their retention is crap.
But yeah...very valid question, but very potentially awkward. It also depends whether you want to know this sorority specifically or the campus in general. If it's the latter, you could ask the Greek life office.
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