There is no clear, 'this is it" signal, as far as I can tell. But, I have seen people who practice try to signal on apps and what not. They usually reference 'the knot'. I have no clue if they're serious.
I believe there are a lot of people curious, interested or just fascinated by zoophilia or bestiality. Some of those people might be willing to or have experimented. But, I think there are only a small group of people who regularly practice. And most of them will be very, very careful about it. The Internet makes it easier for people to keep the desire to reach out and find others in a real world setting limited, and people find one another here, after, I assume a long period of building trust.
Although, I wonder if some people can't help but put a signal out there, just to let those who are like minded know, or maybe just to have a signal out there, and have so many people cluelessly see it. It's always the women that capture my imagination, there's one who i've seen driving a Jeep, with paw print taillights, and a decal in the back tire cover. The paw print tattoo, you see from time to time. And, then again, these women could just like wolves.
I would bet people do try to signal others, whether on purpose, or unconsciously. And, likely that would similar to the way you might to try to safely do it.