For about the 4357921544682067221168712165874262652483268740658th time, unless, with your very own little eyeballs Mk I, you see someone with an animal's dick inserted into one of their bodily orifices, or their dick inserted into an animal's orifice, there is simply NO WAY TO KNOW that isn't 100% pure guesswork and/or wishful thinking on your part.
And even if you DO see such a thing, there's no way to know if they're just getting their rocks off, or they actually care about the critter involved to some degree greater than "it has a dick it can cram into me/a hole I can cram my dick into".