If you really want to make sure there are no cats on your flight (eg. if you are deathly allergic and the airline will not assure you that the flight will be fur-free), simply pay the fee and reserve the spots as though you were taking your own cats.

If you're allergic, but not deathly allergic, please take your anti-histamines and let the passengers with pets travel with the peace-of-mind that their companion is not suffering the brutal conditions of the cargo-hold. Countless animals die, go missing, suffer injuries, or simply endure intense stress traveling the cargo hold. I for one am grateful that the airlines allow pets as discrete carry-on.