That is an excellent question...
scent sac issues are not very common in cats but they do happen.
A thread about my Dexter scent sac issue is linked below.http://www.thecatsite.com/forums/sho...ght=scent+sacs
To answer your main question, if a cat scoots, licks the anal area frequently or you notice swelling, or a unusual scent in the anal area, have a vet take a look see.
I can tell when Dexter's scent sac fluid is becoming to thick to express on it's own, when his energy level becomes depressed and his anal area looks a bit swollen on the plugged side.
Hopefully you will never encounter this frustrating problem.