Black Beauties,
I'd replace all of them on sight.

Kill 'em all. They are junk by now, just like earlier wax/paper caps, for
any serious application.

It takes less time to simply replace one than it does to pull it out, test
it, and either re-install it or replace it.

You won't regret it, and you have nothing to lose. Unless you're one of
those vintage electric guitar nitwits who wants to find an original
"bumblebee" cap or two to provide that tasty "period" sound (i.e.,
distortion resulting from badly leaking capacitors, which certainly wasn't
what the guitar sounded like when new).

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html