Be upfront but not pushy.
Also, scope them out. Ask their opinion on zoophilia for example. Maybe not straight at first, go roundabout way, like tell you like animals, you're an animal lover, talk about animal rights laws and how they feel about them. Also, if they are a real over-the-top "doggy parent" or similar, seeing animals as babies, they probably are not good to tell about this, ever.
Stuff I've said to scope people out have been like "I don't think an animal like a horse can be hurt by sexual contact with humans, have you seen the size of a horse penis lol?", "That one guy got arrested for having sex with a dog. I think it was wrong, the dog was healthy and outgoing and did not seem hurt at all", "I've seen so many dogs humping legs, I can't seem to think letting a dog mount you would hurt them at all"
You'll know how to scope out your partner the best, you talk to them often (at least I do hope you do) so you know what a low-level scope-out question would be.
I've told about 10~ of my partners about being a zoo, and all were fine with it. A couple of them wanted to indulge in it with me, too.
Being honest and real is the way to go, all other fetishes I have, I just put right on the table. I view it as presenting my lovers with a gift basket full of goodies they can pick from! But this question is so taboo and so often debated on the morality of it, that I reserve it for a partner that I have a steady relationship with already and have felt out specifically for this question.
