The only way to be sure is
measure leakage. Time consuming. An initial sorting with a
fair degree of confidence might rely on cap value (as noted
previously), & physical size vs voltage rating (paper caps
tend to be larger for high voltages). But still the gray-area
ones need testing to sort them.
Once you've started testing,
you probably find units of same maker & shape/size that
are all bad or all good & can sort those faster based on
experience.
Is it worth your time? If
not, maybe it's worth it to someone else.
Wayne
WB4OGM